The United States dramatically raised the alarm Friday over Ukraine, warning that a Russian invasion starting with civilians caught under aerial bombing could begin in days and telling US citizens to leave within 48 hours.

An attack by the more than 100,000 Russian troops currently massed next to Ukraine “could occur any day now,” White House National Security Advisor Jake Sullivan told reporters in Washington.

Dismissing speculation that the Kremlin would never trigger the crisis while the Beijing Olympics were still underway in close Russian ally China, Sullivan said such an attack “could occur” before the Games end on February 20.

The scenario of an imminent attack is “a very, very distinct possibility,” Sullivan said.

While stressing that it was not yet known whether President Vladimir Putin had taken a decision, saying “we can’t predict the exact determination,”

Sullivan made clear that the United States was bracing for the worst, including a “rapid assault” on the capital Kyiv.

“If a Russian attack on Ukraine proceeds, it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians,” he said. “Any American in Ukraine should leave as soon as possible, and in any event in the next 24 to 48 hours.”

Sullivan spoke shortly after Biden and six European leaders, the heads of NATO and the European Union held talks on the worst crisis between the West and Russia since the end of the Cold War.