Hundreds of migrants seek to storm the border from Belarus into Poland, escalating a crisis along the EU's eastern border that has been simmering for months.
Poland has blocked a bid by hundreds of migrants to illegally enter the country from Belarus, warning of an "armed" escalation as thousands more massed near the border.
Poland said that "interior ministry forces and soldiers managed to stop the first mass attempt to breach the border" on Monday.
"Migrants have set up a camp in the Kuznica region. They are constantly guarded by Belarusian services," the defence ministry tweeted.
Polish government spokesman Piotr Muller told reporters that a further 3,000 to 4,000 migrants were massing near the border.
"We expect that there may be an escalation of this type of action on the Polish border in the near future, which will be of an armed nature," he added.
Many of the migrants seeking entry into Poland are desperately fleeing war and poverty-wracked countries in the Middle East.
They say they are stuck between a rock and a hard place, with the Belarusian side refusing to allow them to return to Minsk and fly home, while Poland does not let them cross and make asylum claims.
Muller blamed "people linked to Belarusian special services" for organising the breach.
European Union says Belarusian President Alexander Lukashenko has encouraged the migrant flow in retaliation for existing EU sanctions imposed over its dismal human rights record.
European Commission President Ursula von der Leyen called for new sanctions on Monday.
“The instrumentalisation of migrants for political purposes by Belarus is unacceptable,” Von der Leyen said in a statement after holding phone conversations with the leaders of European Union countries bordering the country.
NATO on Monday also hit out at Minsk, accusing the government there of using the migrants as political pawns.
The US State Department also called on Belarus to "immediately halt its campaign of orchestrating and coercing irregular migrant flows across its borders into Europe".
Lukashenko has denied the accusations levelled against his regime.