KYIV, Ukraine (AP) — Russia announced massive nuclear drills while Western leaders grasped Friday for ways to avert a new war in Europe amid soaring East-West tensions, after unusually dire U.S. warnings that Moscow could order an invasion of Ukraine any day.

Immediate worries focused on the volatile front lines of eastern Ukraine, where an upsurge of recent shelling tore through the walls of a kindergarten and basic communication was disrupted. Western officials, focused on an estimated 150,000 Russian troops posted around Ukraine’s borders, fear the long-simmering conflict could provide the spark for a broader war.

The drumbeat of warnings that a larger conflict could start at any moment continued Friday after U.S. President Joe Biden warned that Washington saw no signs of a promised Russian withdrawal — but instead saw more troops moving toward the border with Ukraine.

Defense Secretary Lloyd Austin said the U.S. believes Russia could launch an attack “any time” and also said he still had seen no sign of the promised Russian pullback. He will hold a call Friday with Russia’s Defense Minister Sergei Shoigu.

Even as Russia claimed to be pulling back troops from extensive military exercises that had sparked fears of invasion, the Kremlin sent a reminder to the world that it has one of the world’s biggest nuclear arsenals, by announcing drills of its nuclear forces for the weekend. The muscle-flexing overshadowed Russian offers this week of continued diplomacy to defuse the Ukraine crisis.

NATO allies are also flexing their might, beefing up military forces around eastern Europe, but insist the actions are purely defensive and to show unity in the face of Russian threats.

The U.S. announced the $6 billion sale of 250 tanks to Poland, a NATO member that has been occupied or attacked by Russia over past centuries. Announcing the deal, Austin said Russia’s military buildup had only reinvigorated NATO instead of cowing it, as Moscow had hoped.

Meanwhile, world leaders meeting at the Munich Security Conference warned that Europe’s security balance is under threat. Germany’s Foreign Minister Annalena Baerbock said that the situation is “calling into question the basic principles of the European peace order.”

“Even steps, millimeters toward peace are better than a big step toward war,” she said.

Moscow has denied any intention of attacking its neighbor, and Russian Foreign Minister Sergey Lavrov mocked the Western warning of an imminent invasion as “fakes” that “cause a smile” in remarks broadcast Friday.

Despite the Russian denials, Washington and its allies are concerned the longtime separatist conflict simmering in eastern Ukraine could provide an excuse for an invasion, though they have not provided details.

With tensions already at their highest level since the Cold War, the Russian military announced that President Vladimir Putin will monitor a sweeping exercise of the country’s nuclear forces Saturday that will involve multiple practice missile launches — a stark reminder of the country’s nuclear might amid the showdown with the West.

While the Kremlin insists it has no plans to invade, it has urged the West to keep Ukraine out of NATO and roll back alliance forces from Eastern Europe — demands roundly rejected by Western allies.

Biden planned to speak by phone Friday with trans-Atlantic leaders about the Russian military buildup and continued efforts at deterrence and diplomacy.

U.S. Secretary of State Antony Blinken revealed some conclusions of U.S. intelligence in Thursday’s speech at the U.N. Security Council, warning that Russia could create a false pretext for an invasion with a “so-called terrorist bombing” inside Russia, a staged drone strike, “a fake, even a real attack … using chemical weapons.” He charged that invasion would open with cyberattacks, along with missile strikes and bombs across Ukraine, describing the entry of Russian troops and their advance on Kyiv, a city of nearly 3 million, and other key targets.

Despite the stark U.S. warnings, Ukrainian officials sought to project calm, with Oleksii Danilov, head of the National Security and Defense Council, saying late Thursday that there were no signs a massive Russian invasion was imminent.

“We don’t undermine the threat in any case, but the possibility of escalation is considered to be relatively low regarding large-scale invasion of Ukraine,” Ukraine’s Defense Minister Oleksii Reznikov told lawmakers Friday.

Nevertheless, U.S. and European officials were on high alert for any Russian attempts at a so-called false flag operation, according to a Western official familiar with intelligence findings. Ukrainian government officials shared intelligence with allies that suggested the Russians might try to shell the areas in the Luhansk region controlled by Moscow-backed separatists on Friday morning as part of an effort to create a false reason to take military action, according to the official who was not authorized to comment publicly.

The area saw a sharp spike in shelling Thursday, with monitors from the Organization for Security and Cooperation in Europe reporting more than 500 explosions before the tensions eased in the evening. Ukrainian authorities and separatists traded accusations of violations of a shaky truce in the nearly 8-year-old conflict in Ukraine’s eastern industrial heartland, called Donbas. The conflict erupted shortly after Russia’s 2014 annexation of the Ukrainian Crimean Peninsula and has killed 14,000.

The Ukrainian military command said shells hit a kindergarten in Stanytsia Luhanska, wounding three people, and cut power to half the town. The rebels said nearly 19 houses were damaged by Ukrainian fire.

Early Friday, separatist authorities in the Luhansk and Donetsk regions reported more shelling by Ukrainian forces along the tense line of contact and Kremlin spokesman Dmitry Peskov said the situation is “potentially very dangerous.”

Ukrainian officials charged that the rebels intensified the shelling in the hopes of provoking a retaliatory attack by government forces.

The Ukrainian military chief, Valerii Zaluzhnyi, said that it’s “not planning any offensive operations or shelling of civilians,” adding that “our actions are purely defensive.”

But amid the fears a wider conflict could still come, a flurry of diplomacy is expected this week.

In addition to the call between the Russian and American defense chiefs, Blinken is expected to meet his Russian counterpart next week.

Meanwhile, Putin met Friday with Belarusian President Alexander Lukashenko to discuss the ongoing joint drills in Belarus that borders Ukraine to the north. The massive exercise involving Russian forces moved from the Far East fueled Western fears that they could use it to cut a short way to the Ukrainian capital.