"US citizens in Belarus should depart immediately via commercial or private means," the State Department said on Monday.
The advisory warned the situation in Belarus was "unpredictable" due to an increase in unusual and concerning Russian military activity near the border with Ukraine," the advisory warned.
Previously, the US government had merely advised its citizens to regularly reassess exit plans in case of emergencies.
Travel to Belarus was already advised against - the highest risk category 4 still applies.
Most recently, the State Department had ordered family members of US government employees to leave Ukraine's northern neighbour. Russia is massively increasing its troops in Belarus on the Ukrainian border.
The State Department also revised its travel advice for Moldova - another country bordering Ukraine.
Here, too, the department noted the "unusual and worrying Russian military activities around Ukraine".
Belarus, Moldova and Ukraine are located between the Eastern European NATO states and Russia. For weeks, the US in particular has been warning that Russia could invade Ukraine.
Tens of thousands of Russian soldiers have gathered close to the Ukrainian border. The Kremlin has repeatedly denied allegations that it plans to invade.
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