The Biden administration is urging U.S. citizens in Belarus to depart the country immediately and warned against travel there in a statement published Monday.

The updated travel warning comes after bordering countries Lithuania, Latvia and Poland have stepped up security along the border over concerns about Russian Wagner mercenary forces exiled in the country.  

The State Department, in its warning, encouraged Americans still in Belarus to depart the country immediately and categorized the country as a Level 4 risk, the highest security warning. 

Concerns have grown among NATO nations after reports that members of the Wagner paramilitary group, supported by Russia, were in Belarus heading toward the border with Poland.

Since Wagner Group financier Yevgeny Prigozhin's abortive mutiny in June, thousands of Wagner mercenaries have reportedly been relocating to Belarus, taking the Kremlin up on its offer for pardons in exchange for exile.

Czech police stopped a Russian tennis player from entering the country ahead of the WTA Prague Open tournament, organizers said on Friday, as a new government resolution banning athletes from Russia or Belarus caused the event to scratch other competitors.

The Prague Open starts on Monday and was expected to see a handful of Russian and Belarusian players, including Evgeniya Rodina of Russia or Aliaksandra Sasnovich of Belarus, competing as neutrals, without any national flag or symbol.

Officials from the Defense Intelligence Agency said they had “no reason to doubt” Russian President Vladimir Putin’s claims last month of transferring a first wave of nuclear weapons to Belarus.

Putin originally announced his plans in March to deliver a nuclear-capable Iskander-M missile system to its ally Belarus by July, and senior DIA officials told reporters on Friday “that they have had some success” in transferring the weapons, according to CNN.

Fighters from the Wagner group have arrived in Belarus from Russia, Ukrainian and Polish officials said on Saturday, a day after Minsk said the mercenaries were training the country's soldiers southeast of the capital.

"Wagner is in Belarus," Andriy Demchenko, a spokesman for the Ukrainian border agency, said in a statement on the Telegram messaging app. He said the movement of "separate groups" from Russia had been observed in Belarus.

Some Wagner fighters have been in Belarus since at least Tuesday, two sources close to the fighters told Reuters.

A year after tennis players from Russia and Belarus were banned from playing at Wimbledon following the invasion of Ukraine, organizers have accepted entries from players from the two countries for the 2023 grand slam tournament if they have complied with “appropriate conditions” and compete as neutral athletes.

Though tournament organizers have been keen to emphasize that they “continue to condemn totally Russia’s illegal invasion,” chatter around the war at Wimbledon remains a touchy subject.