A large crowd of migrants was gathering at a shut border crossing between Poland and Belarus on Monday, videos released by Poland's border guards and defense ministry showed.
"More and more groups of migrants are being brought to the Kuznica border crossing by Belarusian forces," the defense ministry said on Twitter, as videos appeared to show hundreds of migrants in front of lines of Polish police and soldiers.
Deputy Interior Minister Maciej Wasik said there were "thousands of migrants" at the crossing.
"Polish forces are prepared for any scenario," he said.
But border guard spokeswoman Anna Michalska said there were "several hundred people."
"We expect that there will be an attempt to forcefully push through the border. At the moment all our forces that we could direct there are being directed there," she said.
The videos could not be independently verified as Poland has banned journalists from the immediate border area under state of emergency rules brought in as part of efforts to block migrants.
Poland says that between 3,000 and 4,000 migrants, most of them from the Middle East, are currently camped out along the border in a crisis that has pitted Western countries against Belarus and its ally Russia.