"A group of a few hundred people are heading for the Kuznica border crossing. One group has already tried to cross the border, but the border guards have arrested some of these people," Polish government spokesman Piotr Muller told reporters in Warsaw.

Muller blamed "people linked to Belarusian special services" for organising the migrant breach. He added that a Polish government crisis unit was due to meet later on Monday.

NATO slammed as "unacceptable" Belarus's alleged tactical use of migrants to put pressure on the EU, saying it was worried about "escalation" on the border with Poland.

- Deaths -

The European Union accuses Belarusian strongman Alexander Lukashenko of facilitating a wave of migrants and refugees, mainly from the Middle East, trying to enter the EU in retaliation for sanctions imposed by Brussels.

Minsk has reportedly issued special visas allowing migrants to fly into Belarus.

The sanctions were imposed after Belarus diverted a flight between EU capitals in May to arrest a dissident journalist and his girlfriend on board, and for a brutal crackdown on the opposition.

Lukashenko has denied the charges.

Poland has sent thousands of soldiers to the border area, enforced a state of emergency in the region complete with a media blackout, built a razor-wire fence and approved the construction of a wall.

Caught in the middle, migrants often report being forced to cross the border by Belarusian officials and then being pushed back into Belarusian territory by the Polish authorities.

At least 10 migrants have died so far in the region, including seven on the Polish side of the border, according to the Polish daily Gazeta Wyborcza.

- 'Prepared for any scenario' -

Poland's defence ministry tweeted a video taken from a helicopter showing hundreds of migrants gathered on the Belarusian side near the Bruzgi-Kuznica border crossing.

Earlier Stanislaw Zaryn, a spokesman for Poland's intelligence services, warned the migrants would "try to enter Poland en masse", adding that "the group is under the strict control of armed Belarusians. They are the ones who decide the direction the group takes."

"We are prepared for any scenario," Poland's interior minister Mariusz Kaminski said in a Monday tweet, adding that Warsaw had beefed up numbers of police and soldiers in the area.

Other videos, which were also published on Twitter by the Nexta Belarusian opposition media outlet, showed groups of hundreds -- if not thousands -- of people in winter clothing, carrying backpacks and walking along a road.

According to a geolocation check by AFP's fact-checking service, one of the videos was taken near buildings in Bruzgi, Belarus, some 1.2 kilometres from Poland's Kuznica border crossing. The signs along the road were in Belarusian.

Belarusian border guards confirmed in a statement that "a large group of refugees... is moving along the highway towards the border with Poland."

Belarusian opposition leader Svetlana Tikhanovskaya blamed Lukashenko for orchestrating a "hybrid attack" and called on the United Nations Security Council to "discuss this crisis" in a Monday Twitter post.