With fears mounting of war in Europe, President Joe Biden prepared to gather Nato allies by phone to discuss the threat to Ukraine, and President Vladimir V. Putin of Russia planned to preside over the launches of nuclear-capable missiles on Saturday in drills that will showcase the country’s most destructive weapons.

Biden’s urgent diplomacy and Putin’s show of strength illustrated the spiralling tensions between Russia and the West, which threaten to spill into all-out conflict. Fighting escalated on Thursday along a volatile front line between Ukrainian forces and Russia-backed separatists in eastern Ukraine, the type of situation that the Biden administration says Moscow could use as a pretext to launch a wider invasion.

The Biden administration has said it believes Russia is poised to invade Ukraine within days. Although Moscow insists that it has no such plans, it has vowed to mount “a tough response” if the US and its Nato partners do not roll back their presence in eastern Europe.

In a demonstration of strength, Russia will conduct major drills this weekend that will include the launch of ballistic and cruise missiles, the defence ministry said.

Russia’s drills will test its strategic nuclear forces, which include the land-based launchers, bombers and warships used to deliver nuclear weapons. They will involve the Black Sea Fleet, which has been engaged in large-scale exercises in the region bordering Ukraine. Putin will preside over them from a “situation centre,” the Kremlin said.

“Even test launches of this type are, of course, impossible without the head of state,” Dmitri S. Peskov, the Kremlin’s spokesman, told reporters. “You all know about his famed ‘black briefcase’, ‘the red button’ and so on.”

The defence ministry said the drills were planned in advance, and Peskov denied that they were intended to raise tensions. But they will come at a critical juncture in the standoff over Ukraine.

As high-level diplomacy continues to compete with the military maneuvering, an uneasy calm settled over eastern Ukraine early on Friday after a night punctuated by explosions and bursts of gunfire in as many as 30 villages and towns along a 250-mile stretch of land separating Ukrainian and Russia-backed forces. Although periodic exchanges of gunfire are not uncommon in the grinding, eight-year trench war, that violence was of a heightened scale.

As the events play out, officials in Russia, the US and in Ukraine are trying to shape the narrative.

US officials said they were “watching closely” out of concern that Russia could use violence in eastern Ukraine as a pretext to invade. Secretary of state Antony J. Blinken told the UN Security Council on Thursday that Russia planned to “manufacture a pretext for its attack”, possibly with a “so-called terrorist bombing” or “a fake, even a real attack” with chemical weapons.

New York Times News Service