MOSCOW (AP) — Separatist leaders in eastern Ukraine ordered a full military mobilization Saturday amid a spike of violence in the war-torn region and fears in the West that Russia might use the strife as a pretext for an invasion.

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 an “immediate threat of aggression" from Ukrainian forces, accusations that Ukrainian officials vehemently denied earlier.

“I appeal to all the men in the republic who can hold weapons to defend their families, their children, wives, mothers," Pushilin said. ”Together we will achieve the coveted victory that we all need."

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 humanitarian convoy hit by shelling, has risen in recent days. A car bombing Friday in the city of Donetsk also sharpened the sense of alarm.

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.

Ukraine’s military said shelling killed a soldier Saturday in the government-held part of the Donetsk region and that separatist forces were placing artillery in residential areas to try and provoke a response.

On Friday, the rebels began evacuating civilians to Russia with an announcement that appeared to be part of their and Moscow’s efforts to paint Ukraine as the aggressor.

U.S. President Joe Biden said late Friday he was now “convinced” that Russian President Vladimir Putin has decided to invade Ukraine and assault the capital, Kyiv.

Biden, who for weeks had said the U.S. was not sure if Putin was determined to send troops into the neighboring country, cited American intelligence as the source of his ominous assessment.

“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 Ukraine, and pressed Putin to reconsider.

A U.S. 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 U.S. 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 between Moscow and Washington remain open: the American 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. Violations of a 2015 ceasefire agreement, including shelling and shooting along the line of contact, have been common.

However, violence has escalated in recent. A bombing struck a car outside the main government building in the rebel-held city of Donetsk on Friday, 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. 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. The center 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.

By Saturday morning, the separatists in the Luhansk and Donetsk regions, which form Ukraine's industrial heartland known as the Donbas, said more than 6,600 residents of the rebel-controlled areas had been evacuated to Russia.

Separatist officials announced plans Friday to evacuate hundreds of thousands of people. Russia 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.

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. U.S. authorities have alleged that the Kremlin's effort to come up with an invasion pretext 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 Russian 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.

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.