Poland has warned of a possible “armed” escalation on its border with Belarus, fearing its neighbour may try to provoke an incident with hundreds of migrants seeking to cross into the EU.
Extra troops have been deployed after desperate crowds tried to cut a barbed-wire border fence.
Poland, the European Union and Nato say Belarus is orchestrating the problem, a claim denied by its disputed leader.
Poland says it is shutting a major border crossing at Kuznica.
EU and Nato members Poland, Lithuania and Latvia have all seen a surge in the number of people trying to enter their countries illegally from Belarus in recent months. Many of them have come from the Middle East and Asia.
Overnight temperatures at the border have slumped below zero and several migrants have already died in recent weeks. Activists say the migrants – many of them young men but also women and children – are being used as pawns in a political game between non-EU Belarus, run by authoritarian leader Alexander Lukashenko, and its neighbours.
Polish government spokesman Piotr Muller said up to 4,000 migrants had gathered near Poland’s eastern border and told reporters that at some point they expected “an escalation… which will be of an armed nature”.
Polish TV said many of the migrants gathering near Kuznica were accompanied by armed individuals with dogs.
The head of Poland’s national security department, Stanislaw Zaryn, said the migrants were under the control of Belarusian armed units. “Belarus wants to cause a major incident, preferably with shots fired and casualties,” deputy foreign minister Piotr Wawrzyk said earlier.
The Belarus defence ministry rejected the Polish statements as unfounded and unsubstantiated, and accused Warsaw of violating agreements by moving thousands of troops to the border.
The EU alleges the Belarusian leader is provoking the influx in retaliation against the bloc’s sanctions.
It imposed the measures after Mr Lukashenko’s crackdown on mass protests following last year’s widely discredited presidential election, and the arrest of a dissident journalist on board a Ryanair flight that was forced to land in Minsk.
European Commission President Ursula von der Leyen said the EU would look at extending sanctions to include “third-country airlines” involved in flying migrants to Belarus.
Mr Lukashenko has repeatedly warned his government will no longer stop the crowds, accusing border guards in the neighbouring EU nations of being violent towards migrants.
Germany on Tuesday urged the EU to “take action” to help Poland secure its borders, while the Nato military alliance said the Belarusian authorities were using migrants as political pawns.