Poland has said it had repelled an attempt by hundreds of migrants to illegally cross the border with Belarus.
It warned that thousands more were on the way and future attempts to breach its frontier could be "armed in nature".
The US called on Belarus to stop what it described as an orchestrated influx of migrants, while the EU called for fresh sanctions against the regime there.
The EU accuses Belarusian leader Alexander Lukashenko of facilitating the wave of migrants and refugees, mainly from the Middle East, in retaliation for sanctions already imposed by Brussels.
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NATO also condemned Belarus, accusing the government there of using the migrants as political pawns.
"Interior ministry forces and soldiers managed to stop the first mass attempt to breach the border," Poland's defence ministry tweeted.
"Migrants have set up a camp in the Kuznica region. They are constantly guarded by Belarusian services."
Polish border guards posted video footage on Twitter showing migrants using wire cutters and branches to breach a razor-wire border fence as Polish officials in riot gear looked on.
Kuźnica: próba sforsowania polskiej granicy pic.twitter.com/kqCkLPdsiK
— Ministerstwo Obrony Narodowej 🇵🇱 (@MON_GOV_PL) November 8, 2021
Polish government spokesman Piotr Muller told reporters that 3,000-4,000 migrants were near the Polish 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.
Mr Muller blamed "people linked to Belarusian special services" for organising the breach. A Polish government crisis unit was handling the response.
NATO denounced Belarus's use of migrants to put pressure on the EU as "unacceptable", expressing concern about "escalation" on the border with Poland.
The EU vowed action in the face of what it said was a "hybrid threat".
In a statement, European Commission President Ursula von der Leyen called on member states to impose new sanctions against Belarus.
The use "of migrants for political purposes is unacceptable", she said in a statement. The EU would also look at how to sanction "third-country airlines" that brought migrants to Belarus, she added.
Belarus has reportedly issued special visas allowing migrants to fly into Belarus from Iraq and other Middle Eastern states.
A statement from the US State Department called on the regime in Belarus to "immediately halt its campaign of orchestrating and coercing irregular migrant flows across its borders into Europe".
Mr Lukashenko has denied the accusations levelled against his regime by the West.
Belarus border official Anton Bychkovsky told the country's BelTA news agency: "All these people, including women and children, do not pose a threat to security and do not behave aggressively.
"According to the refugees, they gathered together in such a large group to prevent their forced expulsion by Poland, as well as to draw the attention of the international community to non-observance of human rights by Poland.".
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, then being pushed back into Belarusian territory by Polish authorities.
At least ten migrants have died in the region, seven of them on the Polish side of the border, according to the Polish daily Gazeta Wyborcza.
"We're prepared for any scenario," Poland's Interior Minister Mariusz Kaminski tweeted, adding that Warsaw had increased the numbers of police and soldiers in the area.
