White House National Security Adviser Jake Sullivan told reporters Friday that U.S. officials believe a Russian invasion of Ukraine is imminent, pending a decision by Russian President Vladimir Putin to pull the trigger on war.

“We continue to see signs of Russian escalation, including new forces arriving at the Ukrainian border. As we’ve said before, we are in the window when an invasion could begin at any time should Vladimir Putin decide to order it,” Sullivan said during a White House press briefing.

The briefing was held shortly after PBS foreign affairs and defense correspondent Nick Schifrin reported that U.S. officials were certain that Putin has decided to invade Ukraine and has communicated that decision to the Russian military.