A large explosion hit the center of the separatist-held city of Donetsk in East Ukraine, Reuters reported late Friday, citing Ria News Agency. A car was blown up near a government building in Donetsk, Russian-backed separatists said in a statement. Russia's Interfax news agency reported that nobody was injured after the incident.

The reported explosion comes amid soaring tensions over a possibility that Russian forces may be preparing to further invade Ukraine as Russian-backed separatists in the east of the country have begun to evacuate civilians.

The United States earlier on the same day accused Russia of seeking a pretext to invade Ukraine, as shellfire rang out in the east of the country and Moscow-backed rebels ordered a civilian evacuation of their breakaway enclaves.

An Agence France-Presse (AFP) reporter near the tense front between government forces and rebel-held territory in the Lugansk region heard the thud of explosions and saw damaged civilian buildings on Kyiv's side of the line.

But in the rebel-held areas of Donetsk and Lugansk, separatist leaders accused Kyiv of planning an offensive and government forces of carrying out sabotage, in what U.S. Secretary of State Antony Blinken dubbed a strategy of "creating false provocations."

Amid concern that Moscow would take the purported Ukrainian offensive as a pretext to launch its own intervention, Ukraine's foreign minister furiously denied the allegations.

"We categorically refute Russian disinformation reports on Ukraine's alleged offensive operations or acts of sabotage in chemical production facilities," Dmytro Kuleba declared on Twitter.

"Ukraine does not conduct or plan any such actions in the Donbas. We are fully committed to diplomatic conflict resolution only."

Women and children first

Videos circulating on Russian-language social media showed sirens sounding in separatist held Donetsk as Moscow-backed militia leaders ordered a civilian evacuation to Russia.

Denis Pushilin, head of the so-called Donetsk People's Republic (DPR), declared: "Women, children and the elderly are subject to be evacuated first.

"The president of Ukraine Volodymyr Zelenskyy in the near future will give the order for soldiers to go on the offensive."

The leader of the neighboring Lugansk separatist region in eastern Ukraine Leonid Pasechnik also urged residents to evacuate to Russia "to prevent civilian casualties."

Meanwhile, all eyes were on Russian President Vladimir Putin's next move as Moscow announced he will oversee a weekend drill of "strategic forces" – ballistic and cruise missiles.

"We are seeing a deterioration of the situation," Putin said at a press conference with his Belarus counterpart Alexander Lukashenko in Moscow.

Russia has demanded that the United States withdraw all forces from NATO members in central and eastern Europe and is turning up the pressure on Ukraine.

Blinken told the Munich Security Conference that what has happened "in the last 24 to 48 hours is part of a scenario that is already in place of creating false provocations, of then having to respond to those provocations and then ultimately committing new aggression against Ukraine."

Russia has denied it has any such plan and claims to have begun withdrawing some of the troops massed on Ukraine's borders.

But Ukraine's defense ministry said 149,000 were still there, and US officials allege that the number is still increasing, amounting to as many as 190,000 if pro-Russian rebels are included.

U.S. President Joe Biden is to hold video talks with Western allies, including the leaders of Britain, Canada, France, Germany and NATO, later on Friday to discuss the crisis.

On Thursday, a shell punched a hole in the wall of a kindergarten in government-held territory near the front line in the Ukrainian village of Stanytsia Luganska.

'Could have been much worse'

The 20 children and 18 adults inside escaped serious injury but the attack sparked international howls of protest.

"It hit the gym. After breakfast, the children had gym class. So, another 15 minutes, and everything could have been much, much worse," school laundry worker Natalia Slesareva told AFP at the scene.

On Friday, part of the village remained without electricity.

The Ukrainian joint command center said the rebels had violated the cease-fire 53 times between midnight and 5:00 p.m. Friday, while the Donetsk and Lugansk separatist groups said the army had fired 27 times in the morning.

"There are no losses among the military personnel of the joint forces as a result of enemy actions," the Ukrainian command center said, accusing the rebels of firing artillery from civilian population areas.

"Ukrainian defenders returned fire to stop enemy activity only in case of a threat to the lives of service members."

The conflict in Ukraine's east has rumbled on for eight years, claiming the lives of more than 14,000 people and forcing more than 1.5 million from their homes.

'Keep a cool head'

Speaking in Parliament, Ukraine's Defense Minister Oleksiy Reznikov insisted Kyiv had "no intention of conducting military operations" against the separatists or Russian-annexed Crimea.

"Our mission is not to do any of the things the Russians are trying to provoke us into doing," Reznikov added.

"We have to push back but keep a cool head."

The Russian defense ministry further upped the ante by announcing that Putin would on Saturday oversee an "exercise of strategic deterrence forces ... during which ballistic and cruise missiles will be launched."

The air force, units of the southern military district, as well as the Northern and Black Sea fleets would be involved.

Russia's aggressive stance has sent diplomatic shock waves through the West, scrambling to counter an unpredictable foe during what has been described as the worst threat to European security since the Cold War.