The U.S. State Department believes the Russian army is now prepared to launch an invasion of eastern Ukraine if President Vladimir Putin decides to pull the trigger, according to a senior adminstration official.

“At this point, they are amassed and they could go at a moment’s notice if Putin gave the go ahead,” the official said.

“Don’t do it,” the official added, in a comment directed at Putin.

Top Ukrainian security officials said Thursday that Russia now has 100,000 troops on its side of the Russia-Ukraine border. Other estimates put the number much lower, around 30,000, but still enough to overpower the undermanned and undersupplied Ukrainian armed forces.

CNN reported Wednesday that U.S. intelligence assessments have increased the likelihood that Russia will invade Ukraine in the past week. This has been based on a number of worrying indicators about the Russian military buildup on the Ukrainian border. “This has shifted our thinking that the likelihood of a further Russian incursion is more probable than it was previously thought to be,” one official told CNN.

Russian forces are currently positioned in and around the cities of Rostov, Kursk, and Belgorod and could try to establish a land corridor from Russian to Crimea by attacking the Ukrainian cities of Kharkiv, Luhansk and Donetsk. Russian agents have already been active in that region of Ukraine.

At Thursday’s State Department press briefing, Deputy Spokeswoman Marie Harf said that the size of the Russian force on the Ukrainian border was changing rapidly.

“We are constantly assessing how many there are, talking to the Ukrainians about that as well. But they are massed on the border,” she said. “And we are concerned about Russia taking further escalatory steps with whatever number of tens of thousands of troops they have there, and have called on them not to do so.”

In a speech in Brussels Wednesday, President Obama said that the U.S. and its allies will continue to progress with sanctions against Russia in response to its aggression in Crimea and its threatening actions toward the rest of Ukraine.

“Together, we are imposing costs through sanctions that have left a mark on Russia and those accountable for its actions,” Obama said. “And if the Russian leadership stays on its current course, together, we will ensure that this isolation deepens. Sanctions will expand, and the toll on Russia’s economy, as well as its standing in the world, will only increase.”