Russia has placed more than 100,000 troops near Ukraine's border and ratcheted up war games in the region.
Moscow denies it is planning an assault, but the United States and its Nato allies fear that Russia is planning for war — and are making preparations of their own.
Here are things to know about the international tensions over Ukraine, a crisis reminiscent of the Cold War.
Russia has increased its pressure on the West, warning that it would quickly take "retaliatory measures" if the US and its allies reject its security demands and continue their "aggressive" policies over Ukraine.
Russia has demanded guarantees that Nato never admit Ukraine and other ex-Soviet nations as members and that the alliance roll back troop deployments in other former Soviet bloc countries, a region Moscow still views as its sphere of interest.
Some of the demands, like the membership pledge, are nonstarters for Nato, creating a seemingly intractable stalemate that many fear can only end in a war.
Russian Foreign Minister Sergey Lavrov said he and other top officials will advise President Vladimir Putin on the next steps after receiving written replies from the United States to the demands. Those answers were delivered Wednesday by US Ambassador to Moscow John Sullivan to Russia's Deputy Foreign Minister Alexander Grushko. The US and its allies have already made clear they will reject the top Russian demands.
Ukrainian officials have sought to calm nerves as tensions escalate.
Ukrainian Foreign Minister Dmytro Kuleba said that while the concentration of Russian troops near Ukraine poses a threat, "their number is now insufficient for a large-scale offensive".
"They are still missing some key military elements and systems to mount a big, full-scale offensive," Kuleba said.
He also noted that causing alarm could be an end in itself. Russia, he said, hopes to destabilise Ukraine by "spreading panic, raising pressure on Ukraine's financial system and launching cyberattacks".
Soldiers and civilians in eastern Ukraine are waiting with helpless anticipation to see if war comes.
They understand that their fate is being decided by politicians in distant capitals. Even politicians in their own capital, Kyiv, have found themselves on the sidelines of several rounds of high-stakes diplomacy that so far have not yielded a breakthrough.
The battle-weary region has been fighting pro-Russia separatist fighters since 2014. It is an area close to where Russia has massed tens of thousands of soldiers in a troop buildup that US President Joe Biden says could mount the largest invasion since World War II.
Presidential advisers from Russia, Ukraine, Germany and France held diplomatic negotiations in Paris amid the growing worries that Russia could invade Ukraine.
The talks took place in the "Normandy format", which helped to ease hostilities in 2015, a year after Putin ordered the annexation of Ukraine's Crimean Peninsula and the Russia-backed insurgency began in eastern Ukraine. Kremlin spokesman Dmitry Peskov said that Moscow expected a "good frank" talk.
Several rounds of high-stakes diplomacy have failed to yield any breakthroughs, and tensions escalated further this week.
British Foreign Secretary Liz Truss has urged European nations to do more to support Ukraine, saying "we would like to see our allies do more to help supply defensive support to Ukraine".
The UK has sent anti-tank weapons to Ukraine in response to Russia massing troops near its border. It has ruled out sending combat troops, but Truss said Britain has "ruled nothing out in terms of sanctions".
"In fact, we'll be legislating to toughen up our sanctions regime and make sure we are fully able to hit both individuals and companies and banks in Russia in the event of an incursion," she told the BBC.
"What's important is that all of our allies do the same, because it's by collective action, by showing Vladimir Putin we're united, that we will help deter a Russian incursion."
Among the Nato allies, Germany in particular has faced calls to help Ukraine more and been faulted for a perception that it is prioritising its economic ties with Russia over Ukraine's security.
Amid that backdrop, German Defence Minister Christine Lambrecht said Germany will deliver 5000 military helmets to Ukraine following a request from the country's embassy. She said that it was "a very clear signal that we stand by your side".
Ukraine's ambassador to Germany welcomed the planned delivery of the helmets but described it as a "purely symbolic gesture". Andriy Melnik told German news agency dpa that "this is only a drop in the ocean".
He added that "Ukraine expects a 180-degree turn from the German government, a real paradigm change".
Germany has said that it won't deliver lethal weapons to Ukraine.
Yet German Foreign Minister Annalena Baerbock insisted Wednesday that the European Union's common goal in the Ukraine crisis "is and remains the defence of Ukraine's sovereignty and territorial integrity."
Like Germans, some Italians are suspected of putting business ties with Russia ahead of solidarity with Ukraine.
On Wednesday, Italian business leaders held a video conference with Russian President Vladimir Putin even though the Rome government asked that the conference be cancelled, or at least that companies with government stakes not participate.
During the call, Putin underlined Italy's role as its third-largest EU trading partner, with $5 billion invested by Italian companies in Russia and $3 billion by Russian companies in Italy.
The president of the Italian-Russian Chamber of Commerce, Vincenzo Trani, said the two countries have always been able to overcome "challenging outside factors in recent years ... through dialogue."
Participants included Marco Tronchetti Provera, CEO of Pirelli tires who is president of the Italian Russian entrepreneurs committee. La Repubblica said the CEO of the oil company ENI and the SNAM energy infrastructure company, which are partially state-owned, cancelled.
Croatia's prime minister sought to defuse an uproar after the country"s president said "Ukraine does not belong in Nato" and that Croatian troops should not be part of any Western response force in case of a war.
President Zoran Milanovic is known for populist-style and undiplomatic statements. His remarks Tuesday angered Ukraine, which summoned Croatia's ambassador in response.
Prime Minister Andrej Plenkovic described Milanovic's comments as "unreal in many ways" and said Croatia, a member of Nato and the EU, will continue to have "friendly and partner" relations with Ukraine.
"I can speak for the Croatian government," Plenkovic said. "Croatia's core position is respect of Ukraine's territorial integrity."
Pope Francis prayed that political leaders put aside personal interests and let dialogue prevail amid concerns of a Russian invasion of Ukraine.
Francis kicked off a daylong prayer for peace in Ukraine on Wednesday with a special appeal at the end of his weekly general audience. "Please, never again war" Francis said. "Let us insistently ask the Lord that in this land may fraternity overcome wounds, fears and divisions."
Noting that Thursday marks Holocaust Remembrance Day, Francis said Ukrainians had suffered tremendous cruelties during World War II.
"They deserve peace," he said.
Clinton, the second-youngest living US president, has had a series of health issues.