Poland said Tuesday that Russia is shifting some short-range nuclear weapons to Belarus, a move Polish President Andrzej Duda said could change the NATO alliance in the region. 

Both Russian President Vladimir Putin and his Belarusian counterpart, Alexander Lukashenko, said last month Moscow had already shipped some of its tactical nuclear weapons to Belarus after announcing the plan in March. 

News of the move came a day after the U.S. Embassy in Belarus issued an alert Monday for all American citizens in the country to leave immediately and warned against traveling there.

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The warning comes after Lithuania closed two of its major border crossings with Belarus over concerns about the Russian mercenary group Wagner that has established operations in the country. 

Moscow said it would maintain control over the warheads it sends to Belarus. Belarus is a critical and strategically important nation, as it borders NATO such as Poland, Lithuania and Latvia in addition to Ukraine. 

Duda made his comments at a joint news conference with visiting Portuguese President Marcelo Rebelo de Sousa.

"I was telling President (de Sousa) about the implementation of the declarations by Vladimir Putin that Russia’s tactical nuclear weapons will be moved to the territory of Belarus," he said. "Indeed, this process is taking place, we are seeing that."

As Russia's war in Ukraine continues, Ukrainians are reportedly ramping up arms production. Meanwhile, Ukraine continues to receive millions in military aid from NATO countries.

Fox News Digital's Bradford Betz contributed to this report.