The White House is reviewing options to deter a feared Russian invasion of Ukraine, including providing more military aid to Kyiv and threatening sanctions, to dissuade Russian President Vladimir Putin from escalating the simmering conflict into a full-blown transatlantic crisis.

The deliberations come as President Joe Biden and his aides prepare for a virtual call with Putin next month, a moment that analysts see as an opportunity to signal the costs of an invasion to the Kremlin but also present a path for reducing tension.

Amid spiking U.S. concern over unusual movements by Russian troops on the Ukrainian border, Secretary of State Antony Blinken embarked Monday on a trip to Europe, where Washington is looking to consolidate a position among allies at a summit with NATO foreign ministers in Latvia. Blinken will then go to Sweden for a meeting of the Organization for Security and Cooperation in Europe, also scheduled to be attended by Russian Foreign Minister Sergey Lavrov.

“We will talk about our assessment of what’s happening on Russia’s border with Ukraine, and we will begin that conversation of what are the options that are on the table, and what is it that NATO as an alliance would like to do together?” Karen Donfried, the State Department’s top official for Europe, said ahead of the trip.

Administration officials are trying to craft an approach that neither appeases Russia nor provokes significant escalation, which is harder now than it was nearly eight years ago, when Moscow annexed Crimea and fueled a separatist war in Ukraine’s east that has left more than 13,000 people dead. The Russian and Ukrainian militaries are more advanced, the West remains divided on how tough to be on Moscow, and Putin has grown increasingly bold about pressing Russia’s claims on Ukraine.

“There has never been a more propitious moment for Putin if he wants to invade Ukraine,” said Fiona Hill, who served as a top Russia adviser in the Trump administration.

The Kremlin denies it is planning an offensive. For weeks, however, U.S. officials have warned publicly and in discussions with allies that they are alarmed about Russian troops near the Ukrainian border. NATO Secretary General Jens Stoltenberg on Thursday said Russia had massed a “large and unusual concentration of forces in the region,” including tanks, artillery, armored units, drones and electronic warfare systems, as well as combat-ready troops. Ukraine says Russia has about 94,000 troops near the border.

The intelligence that worried senior Biden administration officials goes beyond the Russian troop buildup, according to U.S. officials, who declined to be more specific and who, like others, spoke on the condition of anonymity to discuss sensitive matters.

The administration is considering enhanced military aid to Ukraine and weighing potential sanctions or other measures that could be taken before or after an invasion, in addition to reviewing military contingency plans, U.S. officials said.

Washington has also floated the possibility of an in-person summit between Biden and Putin in the first half of 2022, according to people familiar with the matter, a move that might buy time to build unity among allies or revitalize a moribund political process to resolve the military conflict in Ukraine’s east.

The potential meeting was broached by CIA Director William Burns in his visit to Moscow earlier this month, they said. The possible in-person meeting was first reported by the Russian newspaper Kommersant. The White House said it had nothing to announce about a meeting.

Ukraine has shored up its defenses since 2014 with Western help. But Ukrainian Brig. Gen. Kyrylo Budanov, the country’s top military intelligence official, this month told the Military Times that Kyiv is seeking additional air, missile and drone defense systems, as well as electronic jamming devices, to help counter rocket and artillery fire.

U.S. officials have said they aren’t sure if Putin is going to attack, or even whether he has reached a decision, noting he could be moving forces near the Ukrainian border as a bargaining strategy with Western powers. After surprising Washington with a similar buildup last spring, Putin landed his first high-profile summit with Biden.

But comments by Putin and other top Russian officials about Ukraine have sharpened in recent months and that more aggressive rhetoric, combined with the second military buildup, have raised fears the Russian leader may not be bluffing.

“When you say things like ‘Ukraine does not now and has never had a right to exist as a sovereign state, that there is no such thing as the Ukrainian people,’ where does your rhetoric go from there?” a senior Western intelligence officer said. “And where has rhetoric like that led in the past? It has pretty consistently been a prelude to conflict.”

The dilemma, said Andrea Kendall-Taylor, a Russia expert at the Center for a New American Security, “is Putin fundamentally cares more about Ukraine than even the United States does. So how do you deter an adversary when there’s such an asymmetry of interests?’‘

Andrew Weiss, a Russia analyst at the Carnegie Endowment for International Peace, said Putin has failed to achieve his goals in Ukraine, and in fact his aggression has worsened the security situation on Russia’s western border, revitalized NATO and strengthened anti-Russian sentiment in Ukrainian society.

“For a whole host of reasons he’ll never admit that, of course, which is part of the reason he continues to see restoring Ukraine to the Kremlin’s sphere of influence as the single most important piece of unfinished business for Russia’s security and his own legacy,” Weiss said.

On Friday, Ukrainian President Volodymyr Zelensky alleged a group of Russians and Ukrainians were found plotting a coup against him - accusations the Kremlin rejected.

The deteriorating situation presents a new test for the transatlantic alliance, which has embraced Ukraine as a partner rather than an ally. It has provided Kyiv with weaponry, training and support, but stopped short of extending a guarantee of defense that formal NATO membership affords.

Ukrainian officials have warned Moscow could mount a simultaneous multifront invasion from the north, south and east to force a retreat and capitulation by Kyiv. Defense Minister Oleksii Reznikov told The Washington Post that a new Russian invasion would be far bloodier and more costly to Europe than the 2014 operation.

The ambiguity about how far NATO would go to defend Ukraine against Russian military action already has sparked debate in Washington and exacerbated divides within the alliance that the Kremlin has sought to exploit.

Samuel Charap, a Russia analyst at the Rand Corporation, argued the United States, faced with limited ability to coerce Putin, should pressure Ukraine into further implementing the moribund 2015 peace deal known as the Minsk II agreement as a symbolic first move to “put the onus on Moscow to deescalate.”

Ben Hodges, the former commanding general of U.S. Army Europe, said Washington should do the opposite, and apply diplomatic, economic and military pressure on Moscow.

“Nothing in Russia’s history should cause anybody to think for one second that giving in to them will cause them to say, ‘OK, we’re good,’” Hodges said. “I believe they really do only respect strength.”

Kendall-Taylor said the upcoming call is a chance to convey the costs of an invasion and emphasize that it would change the security situation in Europe.

“The U.S. would have no choice but to position more forces in Europe,” Kendall-Taylor said.

U.S. officials say they realize meetings alone are not offramps.

“There’s this tension between getting rid of the crisis in the near term - let’s throw another summit at Putin - and the longer-term imperative,” said one official. “If you offer concessions, what do you teach them and China? You teach them to manufacture crises because you get concessions.”

The Biden administration’s response is also sure to be watched closely elsewhere around the world - perhaps most notably in China, where Beijing’s stance on Taiwan in many ways mirrors Russia’s approach to Ukraine.

The latest challenge over Ukraine comes as Europe grapples with an energy crisis that has highlighted its dependency on Russian gas, and as the continent faces a leadership transition in Germany. Outgoing chancellor Angela Merkel for years took the lead on European diplomacy toward Ukraine.

Hill, a senior fellow at the Brookings Institution, said Europe must “step up to meet this challenge.” Any further incursion by Russia into Ukraine, which has been established as an independent nation for 30 years, she said, poses a “massive challenge to the territorial integrity of every other European state.”

The Washington Post’s Karoun Demirjian contributed to this report.