WASHINGTON—Secretary of State Antony Blinken warned of a looming Russian offensive against Ukraine and proposed a last-ditch diplomatic meeting with his Russian counterpart next week that he said could lead to a summit of key leaders.

Mr. Blinken spoke at a United Nations Security Council meeting Thursday just hours after Russia told the U.S. in a formal document that it might resort to “military-technical measures” if it didn’t receive legally binding security assurances.

President...

WASHINGTON—Secretary of State Antony Blinken warned of a looming Russian offensive against Ukraine and proposed a last-ditch diplomatic meeting with his Russian counterpart next week that he said could lead to a summit of key leaders.

Mr. Blinken spoke at a United Nations Security Council meeting Thursday just hours after Russia told the U.S. in a formal document that it might resort to “military-technical measures” if it didn’t receive legally binding security assurances.

President Biden warned the threat of a Russian invasion of Ukraine was “very high” and said the U.S. believed Russia was fabricating a pretext to invade its neighbor. “Every indication we have is they’re prepared to go into Ukraine,” he said.

And Russia announced that it had expelled the No. 2 U.S. diplomat at the embassy in Moscow. The State Department said Thursday that Deputy Chief of Mission Bart Gorman had departed Russia last week, and said it was weighing a response.

Images show what the Ukrainian government says is a kindergarten damaged by shelling in eastern Ukraine's Luhansk region. Photo: Carlos Barrera/Reuters The Wall Street Journal Interactive Edition

The Kremlin has been seeking a formal guarantee that the North Atlantic Treaty Organization will never admit Ukraine and other former Soviet states as members and will roll back troop deployments on the eastern flank of alliance members to the status quo in 1997, before the alliance admitted Eastern and Central European states.

The U.S. and NATO have said that membership in the alliance will remain open to any country that can meet its criteria for inclusion. The Biden administration has also offered to conclude an agreement that would ban the deployment of offensive missiles and the permanent stationing of combat troops in Ukraine if Russia would agree to similar steps.

“This is a moment of peril for the lives and safety of millions of people, as well as for the foundation of the United Nations charter and the rules-based international order that preserves stability world-wide,” Mr. Blinken said.

A satellite image shows armor and artillery loaded on flatcars in Yelnya, Russia, about 200 miles north of the border with Ukraine.

Photo: Maxar/Associated Press

Mr. Blinken said he had invited Russian Foreign Minister Sergei Lavrov to meet next week in Europe, and proposed further meetings of the NATO-Russia Council, a forum for talks between the alliance and Moscow, as well as the Organization for Security and Cooperation in Europe, a grouping that includes both Russia and Ukraine.

“These meetings can pave the way for a summit of key leaders, in the context of de-escalation, to reach understandings on our mutual security concerns,” Mr. Blinken added.

Russia, which is presiding over the Security Council this month, used the forum to submit a report accusing Ukraine of pursuing the “genocide of the Russian-speaking population of Donbas.”

Russian Deputy Foreign Minister Sergei Vershinin didn’t respond to Mr. Blinken’s proposal for a summit but disputed that Russia is preparing to invade Ukraine. “The day before yesterday, some of the units, after their exercises, were returning to their home bases in Russia,” Mr. Vershinin said.

Russia has more than 150,000 troops near Ukraine, according to U.S. officials. That force includes well-trained ground troops, warplanes, surface-to-surface missiles and amphibious vessels in the Black Sea.

Russian Deputy Foreign Minister Sergei Vershinin addressed the U.N. Security Council meeting on Thursday.

Photo: justin lane/EPA/Shutterstock

Defense Secretary

Lloyd Austin said at NATO headquarters Thursday that Russia’s military preparations were moving ahead.

“We see them fly in more combat and support aircraft. We see them sharpen their readiness in the Black Sea,” Mr. Austin said. “We even see them stocking up their blood supplies.”

“I was a soldier myself not that long ago,” added Mr. Austin, a retired Army general. “I know firsthand that you don’t do these sorts of things for no reason. And you certainly don’t do them if you’re getting ready to pack up and go home.”

Mr. Blinken, who addressed the Security Council before flying to a security meeting in Munich, repeated U.S. concerns that Russia could fabricate a provocation that it could use as a pretext for a military attack that could take place “in the coming days.”

Facing Mr. Vershinin across the horseshoe-shaped Security Council table, Mr. Blinken challenged Moscow to commit to de-escalation.

“State it clearly, state it plainly to the world, and then demonstrate it by sending your troops, your tanks, your planes back to their barracks and hangars and sending your diplomats to the negotiating table,” Mr. Blinken said.

Russia billed the U.N. meeting as an annual review of the Minsk agreements, a pair of deals that sought to bring an end to the fighting between Ukraine and Russia-backed separatists in Donbas.

Yet on Wednesday, Russia passed a report about the alleged crimes committed by the Ukrainian military to Secretary-General António Guterres, who circulated it to Security Council members, a spokesman for Mr. Guterres said.

A senior U.S. official accused Russia of spreading disinformation and said that the planning for Thursday’s meeting is “part of a larger picture” that suggests Russia could be fabricating a pretext for deeper involvement in eastern Ukraine.

Kyiv dismissed the allegations of war crimes. “Russia pulled another propaganda ace up its sleeve, blaming Ukraine for ‘genocide’ in the occupied territories,” said Ukraine’s ambassador to the U.N., Sergiy Kyslytsya. “These are fake and irresponsible allegations by Russia at hiding its responsibility for the occupation of the Ukrainian territories and subsequent human-rights violations there.”

Mr. Blinken said a Russian attack could follow a manufactured pretext in the form of a terrorist bombing on Russian territory, a staged chemical-weapons attack or another scenario.

A senior administration official said Mr. Blinken’s remarks about those possible Russian “false flag” operations were based on newly declassified intelligence reporting.

The Biden administration has been releasing a stream of intelligence information that officials say is designed to pre-empt Moscow’s actions and to control the narrative of the events in Ukraine.

—Ann M. Simmons in Moscow and Vivian Salama in Washington contributed to this article.

Write to William Mauldin at william.mauldin@wsj.com, Michael R. Gordon at michael.gordon@wsj.com and Courtney McBride at courtney.mcbride@wsj.com