The United States has issued some of its starkest, most detailed warnings yet about how a Russian invasion of Ukraine might unfold, and its Western allies went on high alert for any attempts by the Kremlin to create a false pretext for a new war in Europe.

US President Joe Biden on Thursday sounded unusually dire as he warned that Washington saw no signs of a promised Russian withdrawal — but instead saw more troops moving toward the border with Ukraine, indicating Moscow could invade within days.

Also read: UK intelligence claims Russia could invade Ukraine without further warning

“Every indication we have is they’re prepared to go into Ukraine, attack Ukraine,” Biden told reporters at the White House. He said the US has “reason to believe” that Russia is “engaged in a false flag operation to have an excuse to go in,” but he did not provide details.

Western fears focus on an estimated 150,000 Russian troops — about 60% of Russia’s overall ground forces — posted around Ukraine’s borders. The Kremlin insists it has no plans to invade, but it has long considered Ukraine part of its sphere of influence and NATO’s eastward expansion an existential threat. A key demand in this crisis is that NATO promises never to allow Ukraine to join.

Also read: Ukraine crisis: Blinken urges Russia to ‘abandon path of war’ at UNSC

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

The US Secretary of State Antony Blinken revealed some conclusions of US intelligence, part of a strategy designed to expose and pre-empt any invasion planning. The US has declined to reveal much of the evidence underlying its claims.

Also read: Russia threatens ‘military-technical’ response after expelling top US diplomat

Blinken told diplomats at the UN Security Council that a sudden, seemingly violent event staged by Russia to justify an invasion would kick off the assault. Blinken mentioned a “so-called terrorist bombing” inside Russia, a staged drone strike, “a fake, even a real attack … using chemical weapons.”

The invasion would open with cyberattacks, along with missile strikes and bombs across Ukraine, he said. Blinken described the entry of Russian troops, advancing on Kyiv, a city of nearly 3 million, and other key targets.

Also read: Joe Biden says invasion of Ukraine due in ‘days’ as Russia responds to NATO

By Thursday evening, US and European officials were on high alert for any Russian attempts to create a pretext for invasion, 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 Luhansk area in the disputed Donbas region on Friday morning as part of an effort to create a false reason to take military action, according to the official who was not authorised to comment publicly.

Violence spiked in a long-running standoff in that area on Thursday, fueling worries it could provide the spark for wider conflict. The region has already been the site of fighting since 2014 that has killed 14,000.