They are mostly from Iraq, Syria and Yemen, and are now enduring freezing conditions in the hope of crossing into the EU.
The EU has accused Belarus and Russia of encouraging thousands of people to cross into Poland and other members states to spark a humanitarian crisis in the bloc – both countries have repeatedly denied any role.
NATO Secretary-General Jens Stoltenberg said on Monday that NATO did not want to speculate on Russia’s intentions on Ukraine, but added: “We see an unusual concentration of troops, and we know that Russia has been willing to use these types of military capabilities before to conduct aggressive actions against Ukraine.”
Pentagon spokesman John Kirby said the build-up was concerning, with US Defense Secretary Lloyd Austin due to meet Ukrainian counterpart on Thursday to discuss the situation.
Russian-backed separatists took control of Ukraine’s eastern Donbass region in 2014 in a conflict that continues.
Moscow claims the Black Sea waters around Crimea, although most countries consider the peninsula still under Ukrainian sovereignty.