As Western nations fear the start of one of the worst conflicts since the Cold War, U.S. Defence Secretary Lloyd Austin said Russian forces were beginning to “uncoil and move closer” to the border with its former Soviet neighbour.

“Having done this before, I can tell you that that’s exactly what you need to attack and the stance that you need to be in to attack,” Austin said at a news conference on a visit to Lithuania.

Lithuanian Foreign Affairs Minister Gabrielius Landsbergis said he was concerned that if the Kremlin was willing to take Ukraine, Moscow would next target the Baltic states and Poland. Austin vowed Washington would stand with its Baltic allies but declined to be drawn in on whether he would answer Lithuanian calls for additional troops.

Russia ordered the military buildup while demanding NATO stop Ukraine from ever joining the alliance but says predictions it is planning to invade Ukraine are wrong and dangerous. It says it is now pulling back while Washington and allies insist the buildup is mounting.

Russian-backed separatist leaders in Eastern Ukraine earlier declared a full military mobilization, a day after ordering women and children to evacuate to Russia, citing the threat of an imminent attack by Ukrainian forces.

Kyiv flatly denied the accusation, and both it and Washington say increased shelling across the ceasefire line this week is part of Russia’s plan to create a pretext for an invasion of Ukraine.

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

Multiple explosions could be heard on Saturday morning near Donetsk in Eastern Ukraine as more people got on buses to leave, a Reuters witness said. The origin was not immediately clear. Ukraine said earlier that one of its soldiers had been killed.

WATCH | Concerns mount that shelling Eastern Ukraine could be pretext for invasion: 

Concerns mount that shelling eastern Ukraine could be pretext for Russia invasion

Shelling erupted Thursday in a region of eastern Ukraine held by Russian-backed separatists, which the international community fears could be used as a pretext for an invasion. 1:56

The Kremlin said Putin, who pledged to protect Russia’s national interests against what it sees as encroaching threats from the West, was watching the nuclear drills together with Belarusian President Alexander Lukashenko from the situation room in the Kremlin.

Notably, the planned exercise involves the Crimea-based Black Sea Fleet. Russia annexed the Crimean Peninsula after seizing it from Ukraine in 2014. .

Underscoring the West’s concerns of an imminent invasion, a U.S. defence official said an estimated 40 per cent to 50 per cent of the ground forces deployed in the vicinity of the Ukrainian border have moved into attack positions closer to the border.

U.S. President Joe Biden, who has given regular warnings of an impending invasion, said on Friday he now believes the capital Kyiv would be targeted by Russia but that he does not think Putin is even remotely contemplating using nuclear weapons.

Biden said Putin would invade in the coming days. “As of this moment, I am convinced that he has made the decision,” he told reporters.

Nuclear exercises

The Kremlin said Russia had successfully test-launched hypersonic and cruise missiles at sea and land-based targets during the exercises by Russia’s nuclear forces.

Putin sat observing the exercises on screens along with the president of neighbouring Belarus, Alexander Lukashenko, from what the Kremlin described as a “situation centre.”

Austin said the nuclear exercises were stoking concern among defence leaders around the world given that Russia’s military was focused on a massive buildup of forces around Ukraine.

“When you layer on top of that a very sophisticated exercise with strategic nuclear forces, that makes things complicated to the degree that you could have an accident or a mistake,” he said.

The drills follow a huge series of manoeuvres by Russia’s armed forces in the past four months that have included a buildup of troops — estimated by the West to number 150,000 or more — to the north, east and south of Ukraine.

Moscow-based analysts said the exercises were aimed at sending a message to take Russia’s demands for security guarantees from the North Atlantic Treaty Organization seriously after the alliance’s expansion to Russia’s borders since the collapse of the Soviet Union in 1991.

“The signal to the West is not so much ‘don’t interfere,’ but instead designed to say that the problem is not Ukraine and actually much wider,” Dmitry Stefanovich, a research fellow at the IMEMO RAS think-tank, told Reuters.

NATO Secretary General Jens Stoltenberg said on Saturday that Russia knew that the alliance could not meet its demands, which include the withdrawal of NATO forces from former communist eastern European states that have elected to join NATO.

New helicopters and a battle group deployment of tanks, armoured personnel carriers and support equipment have deployed in Russia, near the border, according to U.S.-based Maxar Technologies, which tracks developments with satellite imagery.

The Kremlin also has tens of thousands of troops staging exercises in Belarus, north of Ukraine, that are due to end on Sunday. Lukashenko said on Friday they could stay as long as needed.

In a sign of heightened worry about an invasion, the NATO military alliance relocated staff of its liaison office in Kyiv to a city in western Ukraine and to Brussels. A NATO official told The Associated Press that safety of its personnel was the top concern.

Germany and Austria also told their citizens to leave Ukraine. German air carrier Lufthansa cancelled flights to Kyiv and Odessa, a Black Sea port that could be a key target in an invasion.

“We do not fully know what President Putin intends, but the omens are grim, and that is why we must stand strong together,” British Prime Minister Boris Johnson told officials attending the annual Munich Security Conference on Saturday.

Ukrainian President Volodymyr Zelensky met with U.S. Vice-President Kamala Harris at the conference. Harris called the unfolding events “a decisive moment in history” and warned Russia that it would face “unprecedented” financial costs if it attacked Ukraine.

‘Weapon in their hands’

Russian-backed rebels seized a swath of Eastern Ukraine in 2014, the same year that Moscow annexed the Crimea region from Ukraine after protests toppled its pro-Russian leader. Kyiv says more than 14,000 people have since died in the conflict in the east.

In one breakaway region, Denis Pushilin, head of the self-proclaimed Donetsk People’s Republic, said he had signed a decree on mobilization and called on men “able to hold a weapon in their hands” to come to military commissariats. The Luhansk People’s Republic issued a similar decree.

Russian news agencies said on Saturday that 10,000 evacuees had arrived so far in Russia. The separatist authorities say they aim to evacuate 700,000 people.

At a market in Donetsk, 38-year-old Oksana Feoktisova boarded an evacuation bus with her nine-year-old son and her mother. They were accompanied by Feoktisova’s brother Yuri, who stayed behind in Donetsk.

“They don’t let men on, and I wouldn’t go anyway frankly,” Yuri said. “I’m a reservist in any case. I’m an artillery man by birth…. I’m loyal to my state, to my people.”