Russia's action extends: military exercises, originally set to end Sunday, that brought an estimated 30,000 Russian forces to Belarus, Ukraine's neighbour to the north.

Russia on Sunday rescinded earlier pledges to pull tens of thousands of its troops back from Ukraine's northern border, a move that US leaders said put Russia another step closer to launching what they said was the planned invasion of Ukraine.

Residents of Ukraine's capital filled a gold-domed cathedral to pray for peace.

Russia's action extends what it said were military exercises, originally set to end Sunday, that brought an estimated 30,000 Russian forces to Belarus, Ukraine's neighbour to the north. They are among at least 150,000 Russian troops now deployed outside Ukraine's borders, along with tanks, warplanes, artillery and other war materiel.

The continued deployment of the Russian forces in Belarus raised concern that they could be used to sweep down on the Ukrainian capital, Kyiv, a city of about 3 million people less than a three-hour drive away.

In Kyiv, life outwardly continued as usual for some on a mild winter Sunday, with brunches and church services, ahead of what US President Joe Biden said late last week was an already decided-upon Russian attack.

Katerina Spanchak, who fled a region of eastern Ukraine when it was taken over by Russian-allied separatists, was among worshippers crowded into the capital's St. Michael's monastery, smoky with the candles burned by the faithful, to pray that Ukraine be spared.

“We all love life, and we are all united by our love of life," Spanchak said, pausing to compose herself. “We should appreciate it every day. That's why I think everything will be fine.”

“Our joint prayers will help to elude this tragedy, which is advancing," said another worshipper, who identified himself only by his first name, Oleh.

A US official said Sunday that Biden's assertion that Putin has made the decision to roll Russian forces into Ukraine was based on intelligence that Russian front-line commanders have been given orders to begin final preparations for an attack. The official spoke on the condition of anonymity to describe the sensitive intelligence.

Russia held nuclear drills Saturday as well as the conventional exercises in Belarus, and has ongoing naval drills off the coast in the Black Sea.

The announcement that Russia was reversing its pledge to withdraw its forces from Belarus came after two days of sustained shelling along a contact line between Ukraine's soldiers and Russian-allied separatists in eastern Ukraine, an area that Ukraine and the West worry could be the flashpoint in igniting conflict.

The United States and many European countries have charged for weeks that Russia is trying to create pretexts to invade. They have threatened massive sanctions if it does.

Biden convened the National Security Council at the White House on Russia's military build-up around Ukraine. White House officials released no immediate details of their roughly two hours of discussion.

"We're talking about the potential for war in Europe,” US Vice President Kamala Harris said earlier Sunday at a security conference in Munich, Germany, that saw urgent consultations among world leaders on the crisis. “It's been over 70 years, and through those 70 years ... there has been peace and security.”

European and US officials insisted they were still pursuing what US Secretary of State Antony Blinken depicted as last-ditch diplomacy.

Ukrainian President Volodymyr Zelenskyy on Sunday appealed on Twitter for a cease-fire. Russia has denied plans to invade, but the Kremlin did not respond to Zelenskyy's offer Saturday to meet with Russian President Vladimir Putin.

After a call with French President Emmanuel Macron, Putin blamed Ukraine — incorrectly, according to observers there — for the escalation of shelling along the contact line and NATO for “pumping modern weapons and ammunition” into Ukraine. The Kremlin statement mentioned a cease-fire only in passing and made no mention of Zelenskyy's call for a meeting.

Macron, who has been prominent in European diplomatic efforts with Putin, spoke separately to Zelenskyy and to Biden after his talks Sunday with Putin. Macron's office said both the Ukrainian and Russian leaders had agreed to work toward a diplomatic solution “in coming days and coming weeks.”

Blinken also renewed his offer to meet one-on-one with Russian Foreign Minister Sergey Lavrov next week — on the condition, he said, that Russia had not invaded by then.

“Up to the last minute, there is still an option for him to pull back,” Blinken told NBC's Meet the Press.

The US Embassy in Moscow issued an advisory urging greater caution by Americans in Russia overall. “Have evacuation plans that do not rely on US government assistance,” it warned.

In the eastern Ukraine regions of Lugansk and Donetsk, separatist leaders have ordered a full military mobilization and sent more civilians to Russia, which has issued about 700,000 passports to residents of the rebel-held territories. Claims that Russian citizens are being endangered might be used as justification for military action.

Officials in the separatist territories claimed Ukrainian forces launched several artillery attacks over the past day and that two civilians were killed during an unsuccessful assault on a village near the Russian border. Ukraine's military said two soldiers died in firing from the separatist side on Saturday.

Immediate worries focused on eastern Ukraine, where Ukrainian forces have been fighting the pro-Russia rebels since 2014 in a conflict that has killed some 14,000 people.

“When tension is escalated to the maximum, as it is now, for example, on the line of contact, then any spark, any unplanned incident or any minor planned provocation can lead to irreparable consequences,” Putin's spokesman, Dmitry Peskov, said in an interview that aired Sunday on Russian state television.

On the front lines, Ukrainian soldiers said they were under orders not to return fire. Zahar Leshushun, peering into the distance with a periscope, had followed the news all day from a trench where he is posted near the town of Zolote.

“Right now, we don't respond to their fire because ..." the soldier said before being interrupted by the sound of an incoming shell. "Oh! They are shooting at us now. They are aiming at the command post.”