Polish security forces turned water cannon on migrants who threw rocks from across the Belarusian border, where thousands have gathered in chaotic attempts to reach the European Union, video footage shared by authorities showed on Tuesday.
The crisis has led the EU to prepare further sanctions against Belarus, which it accuses of attempting to destabilise the bloc by pushing migrants across the border illegally.
Footage shared by a Polish government spokesperson and the Ministry of Defence showed a further escalation of the crisis at the border, where migrants have assembled in growing numbers on the Belarusian side in the last week.
"Attention, attention, if you don't follow orders, force will be used against you," said a loudspeaker message directed at migrants throwing objects, according to the footage, which was shown on public broadcaster TVP.
Migrants were also shown throwing bottles and logs at the border fence and Polish soldiers and using sticks to try to break the border fence in the footage shared by the Polish government.
The Interior Ministry said a policeman was seriously injured by an object thrown across the border and was in hospital with a suspected fractured skull.
The EU says Belarus is encouraging migrants to cross the border in revenge for earlier sanctions over a crackdown on protests last year against President Alexander Lukashenko's contested re-election.
Belarus, a close Russian ally, said assertions it had fuelled the border crisis were "absurd".
Poland's Ministry of Defence said in a tweet that Belarusian authorities had given migrants sound grenades to throw at Polish soldiers and border guards.
Poland's government spokesperson said the government was discussing whether to launch formal consultations on the crisis with NATO allies.
"We are preparing for a pessimistic outcome - that this conflict could stretch out for months," spokesperson Piotr Muller told a news conference.
According to Polish authorities, more than 20,000 Polish security service members from the police, border guard and army are reinforcing the border where migrants have gathered near a crossing at the Polish town of Kuznica.
An estimated 4,000 migrants are near the border and many say Belarusian authorities are not allowing them to return to Minsk.
Earlier on Tuesday, Poland's ruling party leader Jaroslaw Kaczynski said armed conflict was not expected at the moment.
"We have a hybrid war, but an actual war, with arms, is not on our horizon. We are facing an unpredictable enemy," Kaczynski told Polish public radio.
In the meantime, Iraq has scheduled an evacuation flight from Minsk on Thursday. So far about 150 to 200 Iraqis already in Minsk have registered to fly home.
Other Iraqis at the border have struggled to register. "We are working on this with the Belarusian authorities," said Iraq's consul for Russia and Belarus, Majid al-Kinani.
"The number is fluctuating, because people are stuck on the Belarusian border with Poland or Lithuania and so far they have not been authorised to go back to Minsk by the Belarusian authorities," the consul said.
In Lithuania, authorities said the situation was under control, with a deputy interior minister saying that they had caught 47 people who had attempted to approach the border.