Russia was staging another show of military might on Saturday with President Vladimir Putin overseeing drills involving nuclear-capable missiles, hours after the United States warned that it was now sure that Moscow is planning to invade Ukraine within days.
Ukrainian President Volodymyr Zelensky meanwhile was headed to Germany to shore up support among Western allies, despite a significant increase in clashes in the country's east in which a Ukrainian soldier was killed.
The dramatic US warnings, increased shelling on the frontlines and evacuation of civilians from Russia-backed rebel regions in Ukraine have come together to raise the fear of a major conflict in Europe to its highest amid weeks of tensions.
The Kremlin insists it has no plans to attack its neighbour, which has angered Moscow by seeking long-term integration with NATO and the European Union.
But the United States insists that with some 150,000 Russian troops on Ukraine's borders – as many as 190,000, when including the Russian-backed separatist forces in the east – Moscow has already made up its mind.
US President Joe Biden said Friday that he was sure Putin had made the call to invade, regardless of warnings that it would trigger huge Western sanctions.
"As of this moment I'm convinced he's made the decision," Biden said in televised remarks at the White House.
Biden said the attack could come in the next "week" or "days" and that targets would include the capital Kyiv, "a city of 2.8 million innocent people."
Ballistic, cruise missiles
Russia has announced a series of withdrawals of its forces from near Ukraine in current days, saying they were taking part in regular military exercises and accusing the West of "hysteria" with claims of an invasion plan.
But Putin has also stepped up his rhetoric, demanding the West take Russian demands for security guarantees seriously, and is now personally overseeing drills involving nuclear-capable missiles.
Moscow is demanding written guarantees that Ukraine will never be allowed to join NATO, and for the US-led military alliance to roll back deployments in eastern Europe to positions from decades ago.
Russia's Defence Ministry said Saturday's "planned exercises" will test launches of ballistic and cruise missiles.
It said the exercises would involve nearly all branches of Russia's Armed Forces, including its aerospace and strategic rocket forces, as well the Northern and Black Sea fleets, which have nuclear-armed submarines.
"Putin, most likely, will watch the exercises from the situation centre," Kremlin spokesman Dmitry Peskov said.
"Such test launches, of course, are impossible without the head of state. You know about the famous black suitcase and the red button," Peskov said in reference to nuclear launch codes.
'Dramatic increase' in clashes
The volatile frontline between Ukraine's Army and separatists in the Moscow-backed breakaway regions of Donetsk and Lugansk has meanwhile seen a "dramatic increase" in ceasefire violations, international monitors from the OSCE said.
Hundreds of artillery and mortar attacks were reported in recent days, in a conflict that has already rumbled on for eight years and claimed the lives of more than 14,000 people.
Ukraine's Armed Forces accused the rebels of a huge new wave of attacks on Saturday, saying there had been dozens of exchanges of fire by 7:00 am (0400 GMT), with one soldier dying from shrapnel wounds.
The rebels, who also accused Ukrainian forces of new attacks on Saturday, declared general mobilisations in the two regions.
On Friday they had announced mass evacuations of civilians into Russia, where the governor of the neighbouring Rostov region on Saturday declared a state of emergency as several thousand crossed the border.
Moscow and the rebels have accused Kyiv of planning an assault to retake the regions, claims fiercely denied by Ukraine and dismissed by the West as part of Russian efforts to manufacture a pretext for war.
Despite the invasion warnings, Zelensky's office said he would not change plans to personally attend Saturday's Munich Security Conference.
The Ukrainian leader will meet with British Prime Minister Boris Johnson, German Chancellor Olaf Scholz and US Vice-President Kamala Harris, his office said.
"Volodymyr Zelensky expects concrete agreements concerning the delivery to our country of additional military and financial support," his office said, adding that he would return to Kyiv later Saturday.
At the conference, Jens Stoltenberg warned that Russia would only get a bolstered NATO on its borders if it invaded.
"If the Kremlin's aim is to have less NATO on its borders, it will only get more NATO," he vowed.
Accusing Russia of a "blatant attempt to rewrite the rules of the international order," EU chief Ursula von der Leyen warned of a "robust package" of financial and economic sanctions against Moscow in case of any aggression.
by Michael Mainville in Moscow with Dmitry Zaks in Kyiv, AFP