(NewsNation Now) — State Department officials are warning any Americans in Ukraine to leave now as the security situation deteriorates amid Russian aggression toward the Eastern European country.

Officials said they didn’t know how many U.S. citizens are still in Ukraine. But some Americans told NewsNation the mood has quickly changed in the past few days. The movement of Russian troops at the two countries’ border has led to fears of an invasion. 

“I’m definitely nervous,” said one American from California, who asked to use the name Ryan for his safety. He has been in Ukraine with his dog, Bandit, for more than a year, but is making plans to leave temporarily. 

“I have a plan to visit a neighboring country in the next few days as a kind of ‘test it out’ kind of thing,” Ryan said.

The Pentagon announced on Monday afternoon it placed 8,500 military personnel on “heightened preparedness to deploy” to bolster NATO forces, stressing no final deployment decisions have been made. 

Pentagon press secretary John Kirby responded to NewsNation in a press conference, saying “what we’re telling these units to do is to be ready to go on a shorter timeline than what they were before. We are not deploying them now.”

That news comes after State Department officials told Americans on Sunday not to expect U.S.-sponsored evacuations — and to leave while commercial flights are available. The Department has given a “do not travel” warning for American tourists to Ukraine and Russia, and is urging Americans in Ukraine to register with them in the event things worsen.

The U.S. embassy will also hold a town hall with Americans in the country Tuesday. 

Uljana Zamaslo — who grew up in New Jersey — says she isn’t taking any chances. Last week, she moved from Kyiv to western Ukraine to be closer to the border. 

“Tuesday morning, I was driving my daughter into school when I got some information via text message. And I turned the car around, went back to the apartment we were renting, and we started packing. We left wednesday at 10 a.m.,” Zamaslo said. 

“I feel really lucky that we left when we did. … I still feel that from my level of comfort, that we left in probably just the right time.”