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CNN  — 

The US State Department on Monday ordered family members of employees at the US Embassy in Belarus to depart the country and warned American citizens against travel there due to an “unusual and concerning Russian military buildup” along Belarus’ border with Ukraine.

The developments come as US officials warn that Russia could mount a major invasion of Ukraine, including from neighboring Belarus. At a UN Security Council meeting Monday, US Ambassador to the UN Linda Thomas-Greenfield said the US had seen evidence that Moscow intends to mass more than 30,000 troops at the Belarus-Ukraine border by early February and that 5,000 troops were already there.

In an updated travel advisory issued Monday, the State Department said that “due to an increase in unusual and concerning Russian military activity near the border with Ukraine, U.S. citizens located in or considering travel to Belarus should be aware that the situation is unpredictable and there is heightened tension in the region.”

“Given the heightened volatility of the situation, U.S. citizens are strongly advised against traveling to Belarus,” the advisory said, adding that “the U.S. government’s ability to provide routine or emergency services to U.S. citizens in Belarus is already severely limited due to Belarusian government limitations on U.S. Embassy staffing.”

Last week, in a move criticized by the Ukrainian government, the State Department authorized the departure of US government employees from the US Embassy in Kyiv and ordered the departure of family members there “due to the continued threat of Russian military action.”

The US has a limited diplomatic presence in Belarus due to restrictions imposed by the government of strongman leader Alexander Lukashenko. Last August, the Belarusian government told the US Embassy in Minsk to reduce the number of staff to five, and in October it forced the embassy to close its Public Diplomacy and US Agency for International Development offices and lay off more than 20 Belarusian staff effective November 20. The US ambassador to Belarus, Julie Fisher, has been forced to serve in neighboring Lithuania.

On Monday, State Department spokesperson Ned Price said the US has “been very clear, including to Belarusian authorities directly, that if it allows its territory to be used for an attack on Ukraine, it would face a swift and decisive response from the United States and our allies and partners.”

“We’ve spoken of the unprecedented nature of sanctions and other economic measures that would befall the Russian Federation in the event of an invasion. Were Belarus – what should be a sovereign, independent country – if that country were to support such an invasion, our response would also be swift and decisive,” he said at a State Department briefing.

At Monday’s UN Security Council meeting over Russia’s escalation toward Ukraine, the Belarusian ambassador to the UN said his country was in support of dialogue but reiterated that it would act in defense of its ally Russia if it were attacked. He also said that Russia and Belarus would be holding joint military activities, which he said “are always purely defensive in nature, and they pose no threat either for our European partners, or our neighboring countries.”