NATO: we stand in solidarity with Poland and other affected allies

"We are deeply concerned about the way the Lukashenko regime is using vulnerable migrants as a hybrid tactic against other countries and he is putting the lives of the migrants at risk," he said. "We stand in solidarity with Poland and other affected allies," Stoltenberg told reporters as he arrived for a meeting with EU defence ministers.

Russian President Vladimir Putin and Belarusian President Alexander Lukashenko discussed the migrant crisis on the Belarusian border with Poland by phone on Tuesday, the TASS news agency cited the Kremlin as saying.

Russia's Foreign Minister Sergei Lavrov on Tuesday accused Poland of "outrageous" behaviour, and the EU leadership of following "double standards".

"The behavior of the Polish side is completely unacceptable. I think the watercannons and tear gas and shots over the migrants' heads towards Belarus state - all of it reflects a desire to hide their own actions. And they can not but understand that they violate all thinkable norms of the international humanitarian law and other international law norms," Lavrov said.

"One of the goals which as we now see is solved by the Polish side - is not to allow for its actions to become publicly known. They just do not allow journalists there. Journalists by CNN, BBC and other western media and television channels - they work on Belarusian side and do not understand why they are not allowed to Polish side," Russian Foreign Minister added.

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.

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.

Migrants threw bottles and wooden logs at Polish soldiers, and used sticks to try to break through the fence, the video showed.

Poland's Border Guard informed that its female officer has been taken to a hospital after she was hit with a rock in the head.

Earlier on, 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 Polish defence ministry said in a tweet that Belarusian authorities had given migrants sound grenades to throw at Polish soldiers and border guards.

An estimated 4,000 migrants are near the border and many say Belarusian authorities are not allowing them to return to Minsk.

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.