Russia is continuing its military buildup around Ukraine, the North Atlantic Treaty Organization’s secretary-general said, and Kyiv put its forces on alert even as Moscow announced that it had begun drawing down some of its troops and released footage of tanks and armored personnel carriers departing Crimea.

On Wednesday, the day on which some U.S. intelligence officials had said a Russian invasion was likely to occur, Ukrainians rallied across the country in a display of solidarity and defiance, raising the national flag...

Russia is continuing its military buildup around Ukraine, the North Atlantic Treaty Organization’s secretary-general said, and Kyiv put its forces on alert even as Moscow announced that it had begun drawing down some of its troops and released footage of tanks and armored personnel carriers departing Crimea.

On Wednesday, the day on which some U.S. intelligence officials had said a Russian invasion was likely to occur, Ukrainians rallied across the country in a display of solidarity and defiance, raising the national flag and singing the anthem in morning ceremonies. The country was rattled by a cyberattack the previous day that targeted the Ministry of Defense and two of its biggest banks, temporarily disrupting payments and showing zero balances on accounts.

“We have heard the signs from Moscow about readiness to continue diplomatic efforts,” NATO Secretary-General Jens Stoltenberg said as defense ministers from the alliance’s 30 member states gathered in Brussels. “But so far, we have not seen any de-escalation on the ground. On the contrary, it appears that Russia continues its military buildup.”

Footage released Wednesday by Russia’s Defense Ministry showed trains ferrying tanks across the Crimean Bridge as Russia said it was moving troops from positions near Ukraine’s border. A NATO official said Moscow’s military buildup around Ukraine continues. Photo: Russian Defense Ministry via AP The Wall Street Journal Interactive Edition

Some 130,000 Russian troops have gathered near Ukraine, including some 30,000 for military drills in Belarus, in recent weeks, prompting the U.S. to shut down its embassy in Kyiv and to evacuate American diplomats and military personnel. Russian President Vladimir Putin said after meeting German Chancellor Olaf Scholz on Tuesday that he had ordered “a partial troop withdrawal” and that he is open to continuing talks with the West about Russia’s security demands and the future of Ukraine.

On Wednesday, Russia’s Defense Ministry said that some of these units had begun leaving Crimea, which Moscow seized from Ukraine and annexed in 2014, for their permanent bases. Some of these bases aren’t far from Ukraine, which means that the redeployed troops will continue to pose a threat.

Ukraine has launched military drills of its own along the borders with Russia and Belarus, as well as on its southern coast, which is vulnerable to Russian amphibious landings. “Everyone who needs to defend Ukraine is doing their jobs. All the troops that needed to be deployed to positions to strengthen dangerous approaches have been deployed, and everyone who needed to receive ammunition has received it,” Ukrainian Defense Minister Oleksii Reznikov said in a televised appearance Wednesday. Aircraft-tracking sites showed military activity in Ukrainian skies, with a U.S. Air Force RQ-4A Global Hawk reconnaissance drone flying in loops along the Ukraine’s borders with Russia and Belarus.

NATO Secretary-General Jens Stoltenberg addressed the Ukraine crisis as alliance defense ministers gathered in Brussels.

Photo: Valeria Mongelli/Bloomberg News

Ukrainian President Volodymyr Zelensky, donning fatigues, arrived Wednesday to inspect exercises in the Rivne region near Belarus, and later planned to visit the front-line troops near Donetsk in Ukraine’s east, according to his office. The forces in Rivne practiced repelling a land attack in a border area with a combined operation, using aviation, air defenses, artillery, Turkish-made Bayraktar TB2 armed drones and Ukrainian-made Stugna-P and Corsar guided missiles, Mr. Zelensky said.

Ukrainian troops also deployed with the antitank missiles that the U.S., the U.K. and other allies supplied in recent weeks, such as the FGM-148 Javelin, NLAW and SMAW M-141. “In just a few days, they have flawlessly mastered the weapons that we received from our foreign partners,” Mr. Zelensky said.

Russia’s military drills in Belarus, which U.S. officials say could be used by Russia to attack the Ukrainian capital from the north, are slated to end on Feb. 20, while Russian navy exercises in the Black Sea are scheduled to last until Feb. 19. Belarus Foreign Minister Vladimir Makey said Wednesday that “not a single service member, not a single piece of equipment” will be left in the country by Moscow once the exercises are completed.

U.S. officials are skeptical of such assurances, saying the drills could be a cover for a military strike on Ukraine. President Biden said Tuesday that a Russian invasion remained “distinctly possible.” Ukrainian officials have also dismissed Russia’s statements on withdrawals, saying that Russian troops could easily move back toward the border—and that Kyiv should be preparing for a permanent threat of a Russian invasion. “Putin wakes up every morning thinking of how to make sure that we don’t exist anymore,” said Oleksii Danilov, the head of Ukraine’s National Security and Defense Council.

The cyberattack that took place Tuesday was the largest denial-of-service attack in Ukrainian history, Mykhailo Fedorov, the minister of digital transformation, said Wednesday. Such attacks knock systems offline by overwhelming them with a flood of activity. “It is clear that it was prepared in advance, and the key goal of this attack is to destabilize, sow panic, do everything so that a certain chaos arises in our country,” he said in a televised briefing.

The Biden administration believes Russian hackers likely have broadly targeted the Ukrainian government, including its military and critical infrastructure networks, to both collect intelligence and to be ready in the case of invasion to launch disruptive cyberattacks, a senior official said.

In recent weeks, Ukraine has been at the center of a flurry of diplomatic activity, with the leaders of France and Germany shuttling between Kyiv and Moscow in an attempt to avert hostilities. These diplomatic efforts focused largely on the Minsk-2 agreements brokered by France and Germany in 2015 that ended major combat between Ukraine and Russian-backed forces in the Donetsk and Luhansk regions. Russia’s interpretation of the accords, which has been rejected by Kyiv, could give Moscow a veto over Ukraine’s key policies.

Ukrainians at Kyiv’s Olympic stadium celebrate a national day of unity.

Photo: Anastasia Vlasova for The Wall Street Journal (2)

While the talks have been fruitless so far, Mr. Putin said after meeting German Chancellor Olaf Scholz in Moscow on Tuesday that he expected Washington, Paris and Berlin to exert “appropriate influence” to pressure Ukraine on Minsk-2.

On the same day, the Russian parliament asked Mr. Putin to recognize the Russian-controlled statelets in Donetsk and Luhansk as independent nations. Mr. Putin’s spokesman Dmitry Peskov said Wednesday that the Russian president has received the request but intends to focus on implementing the Minsk agreements, under which Donetsk and Luhansk would become autonomous areas within Ukraine. Mr. Peskov said that Mr. Putin was in favor of continuing negotiations and welcomed Mr. Biden’s willingness to do the same.

Russian officials mocked American warnings of a Wednesday invasion, calling them hysteria and proof that the U.S. cannot be trusted. “We need to stop believing everything that they say in Washington, especially regarding Ukraine,” Russian Foreign Ministry spokeswoman Maria Zakharova said Wednesday on the YouTube channel of Ukrainian journalist Anatoly Shariy, who is sought by Kyiv for alleged state treason. Russian ambassador to the European Union Vladimir Chizhov

quipped in an interview with a German TV channel that “wars in Europe rarely begin on Wednesdays.”

In a sign of how the crisis is hurting Ukraine’s economy and broader society, two more airlines said they would temporarily halt flights to the country. Emirates Airline’s sister carrier, FlyDubai, and Israel’s national airline El Al suspended services starting on Wednesday, following a similar decision over the weekend by KLM Royal Dutch Airlines. Ukrainian airlines SkyUp and Ukraine International Airlines said earlier this week that they have had to ground some of their fleet because insurance companies canceled coverage.

U.S. officials said earlier this week that the Russian military presence near Ukraine had grown to 105 battalion tactical groups, up from 83 groups earlier this month. Russia has also moved around 500 combat aircraft within range of Ukraine and has 40 combat ships in the Black Sea, according to U.S. officials.

“What we see is that they have increased the number of troops, and more troops are on their way,” said NATO’s Mr. Stoltenberg. “It has been a bit up and down, back and forth, all the way, but the trend over the last weeks and months has been a steady increase in the Russian capabilities close to Ukraine’s borders.”

Amid these warnings, Ukraine’s Mr. Zelensky sought to raise morale and upend the narrative by proclaiming Wednesday as a national holiday. Yellow-and-blue national flags lined Kyiv’s main thoroughfares, and a morning rally at Kyiv’s central sports stadium drew Ukrainians out to sing the national anthem, which starts with the line “Ukraine is not dead yet.”

“We have to show support for our country and our president. This is a difficult time. We’re worried, but we’re trying to keep from panicking,” said Roman Dudiak, 20, a university student who fled his hometown of Donetsk when Russian-controlled forces seized it in 2014. “Russia didn’t attack us yesterday or last week, we’ve been fighting Russia for eight years.”

—Benjamin Katz and Dustin Volz contributed to this article.

Write to Yaroslav Trofimov at yaroslav.trofimov@wsj.com, Thomas Grove at thomas.grove@wsj.com and Ann M. Simmons at ann.simmons@wsj.com