The U.S. State Department is calling on Americans in Russia and Belarus to “leave immediately” as tensions mount over the Russian invasion of Ukraine.

On Monday, the State Department authorized the voluntary exit of non-emergency personnel and family members of diplomats at the embassy in Moscow. A Level 4 travel alert was issued for Russia because of the deteriorating relations over Ukraine and the "potential for harassment against U.S. citizens by Russian government security officials."

The State Department on Sunday urged U.S. citizens in Russia to have plans to leave the country as available flights in and out of the country become fewer after European nations closed their airspace to Russian airlines.

The U.S. embassy in Belarus’ capital Minsk was also shuttered Monday over security concerns related to the war in Ukraine. Belarus has allowed its territory to be used as a launch pad for Russian forces entering Ukraine and several missiles have been fired from its territory against Ukrainian positions since the fighting began.

A decision by Belarus President Alexander Lukashenko to revise the country’s constitution by removing its “non-nuclear” status in a referendum also contributed to Washington’s decision to exit the country.

With that provision removed, it is feared that Lukashenko may request Russia to station nuclear weapons on its territory. In a statement, the U.S. described the referendum as a “sham” that it would not recognize.

“We took these steps due to security and safety issues stemming from the unprovoked and unjustified attack by Russian military forces in Ukraine,” Secretary of State Antony Blinken said in a statement.

Ukrainian cities have been attacked by Russian troops, but they have resisted mightily and inflicted numerous casualties on the invaders.

In a collective response, the U.S. together with its Western allies initiated a massive sanctions response aimed at pressuring Russia’s economy. These sanctions target Russian financial institutions and senior Russian officials, including President Vladimir Putin.

Putin has been undeterred and warned the West that it was making a mistake by trying to push Russia out of the global economy. However, the Moscow Stock Exchange saw one of the worst declines in history after the war began and sanctions ate into the ruble’s value. As of Monday, it remains closed for operations.