Denis Pushilin, the head of the pro-Russia separatist government in Ukraine's Donetsk region, released a statement announcing a full troop mobilization and urging reservists to show up at military enlistment offices.
A similar announcement quickly followed from Leonid Pasechnik, separatist leader in the Luhansk region.
Pushilin cited ``immediate threat of aggression`` from Ukrainian forces, accusations that Ukrainian officials vehemently denied earlier.
The separatists and Ukrainian forces have been fighting for almost eight years. But the violence along the line of contact separating the two sides, including a car bombing in the eastern city of Donetsk and a humanitarian convoy hit by shelling, has risen in recent days.
With an estimated 150,000 Russian troops now posted around Ukraine's borders, the long-simmering separatist conflict could provide the spark for a broader attack.
On Friday, the rebels began evacuating civilians from the conflict zone with an announcement that appeared to be part of their and Moscow's efforts to paint Ukraine as the aggressor instead.
In an ominous assessment, US President Joe Biden said he was now ``convinced'' Russian President Vladimir Putin has decided to invade Ukraine and assault the capital, Kyiv.
Biden, who for weeks had said the US was not sure if Putin had made the final decision, said his judgment had changed, citing American intelligence.
``As of this moment, I'm convinced he's made the decision,'' Biden said. ``We have reason to believe that.'' He reiterated that the assault could occur in the ``coming days.''
Meanwhile, the Kremlin announced that it would conduct massive nuclear drills Saturday, and Putin pledged to protect Russia's national interests against what it sees as encroaching Western threats.
Biden reiterated his threat of crushing economic and diplomatic sanctions against Russia if it does invade, and pressed Putin to reconsider. He said the US and its Western allies were more united than ever to ensure Russia pays a steep price for any invasion.
As further indication that the Russians were preparing for a major military push, a US defense official said an estimated 40% to 50% of the ground forces deployed in the vicinity of the Ukrainian border have moved into attack positions closer to the border.
The shift has been underway for about a week, other officials have said, and does not necessarily mean Putin has decided to begin an invasion. The defense official spoke on condition of anonymity to discuss internal US military assessments.
The official also said the number of Russian ground units known as battalion tactical groups in the border area had grown to as many as 125, up from 83 two weeks ago. Each group has 750 to 1,000 soldiers.
Lines of communication remain open: the US and Russian defense chiefs spoke Friday. Secretary of State Antony Blinken and Russian Foreign Minister Sergey Lavrov agreed to meet next week.
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.
Fears of such escalation intensified amid Friday's violence. A bombing struck a car outside the main government building in the rebel-held city of Donetsk, according to an Associated Press journalist there. The head of the separatist forces, Denis Sinenkov, said the car was his, the Interfax news agency reported.
There were no reports of casualties and no independent confirmation of the circumstances of the blast. Shelling and shooting are common along the line that separates Ukrainian forces and the rebels, but targeted violence is unusual in rebel-held cities.
Adding to the tensions, two explosions shook the rebel-controlled city of Luhansk early Saturday. The Luhansk Information Center said one of the blasts was in a natural gas main and cited witnesses as saying the other was at a vehicle service station. There was no immediate word on injuries or a cause. Luhansk officials blamed a gas main explosion earlier in the week on sabotage.
Overall, monitors from the Organization for Security and Cooperation in Europe reported more than 600 explosions in the war-torn east of Ukraine on Friday.
Separatists in the Luhansk and Donetsk regions that form Ukraine's industrial heartland known as the Donbas announced they were evacuating civilians to Russia.
Pushilin, the head of the Donetsk rebel government, said women, children and the elderly would go first, and that Russia has prepared facilities for them. Pushilin alleged in a video statement that Ukrainian President Volodymyr Zelenskyy was going to order an imminent offensive in the area.
Metadata from two videos posted by the separatists announcing the evacuation show that the files were created two days ago, The Associated Press confirmed. US authorities have alleged that the Kremlin's disinformation campaign could include staged, prerecorded videos.
Authorities began moving children from an orphanage in Donetsk, and other residents boarded buses for Russia. Long lines formed at gas stations as more people prepared to leave on their own.
Putin ordered the government to offer a payment of 10,000 rubles (about $130) to each evacuee, equivalent to about half of an average monthly salary in the war-ravaged Donbas region.
By Saturday morning, more than 6,600 residents of the rebel-controlled areas were evacuated to Russia, according to separatist officials, who have announced plans to evacuate hundreds of thousands of people.
The explosions and the announced evacuations were in line with US warnings of so-called false-flag attacks that Russia could use to justify an invasion.
Around the volatile line of contact, a United Nations humanitarian convoy came under rebel shelling in the Luhansk region, Ukraine's military chief said. No casualties were reported. Rebels denied involvement and accused Ukraine of staging a provocation.
Ukraine denied planning any offensive.
``We are fully committed to diplomatic conflict resolution only,'' Foreign Minister Dmytro Kuleba tweeted.
UN Secretary-General Antonio Guterres said the threat to global security is ``more complex and probably higher'' than during the Cold War. He told a security conference in Munich that a small mistake or miscommunication between major powers could have catastrophic consequences.
Russia announced this week that it was pulling back forces from vast military exercises, but US officials said they saw no sign of a pullback and instead observed more troops moving toward the border with Ukraine.
In other developments, the White House and the UK formally blamed Russia for recent cyberattacks targeting Ukraine's defense ministry and major banks. The announcement was the most pointed attribution of responsibility for the intrusions, which barraged websites with junk data to make them unreachable. Russia rejected the accusations.
The Kremlin sent a reminder to the world of its nuclear might, announcing drills of its nuclear forces for the weekend. Putin will monitor the exercise Saturday that will involve multiple practice missile launches.
Asked about Western warnings of a possible Russian invasion on Wednesday that did not materialize, Putin said: ``There are so many false claims, and constantly reacting to them is more trouble than it's worth.``
``We are doing what we consider necessary and will keep doing so,'' he said. ``We have clear and precise goals conforming to national interests.''