An alliance official termed the Belarusian regime's use of migrants "a hybrid tactic", meaning a combined military-political operation, and said: "NATO stands ready to further assist our allies, and maintain safety and security in the region".

Belarus, allied with Russia, is under EU sanctions for diverting a Ryanair flight between EU capitals in May to arrest a dissident Belarusian journalist and his girlfriend on board, and for a brutal crackdown on the opposition.

Authorities in EU country Poland said on Monday that Belarus was sending hundreds of migrants to its border, where it has built high rows of barbed wire and deployed troops to hinder illegal crossings.

The EU has accused Belarusian strongman Alexander Lukashenko of facilitating the wave of migrants and refugees, mainly from the Middle East, in retaliation for the sanctions.

A Polish emergency law has barred media from reporting along the border, where at least 10 migrants have died, apparently blocked from entering further into Poland and stopped from going back into Belarus.

"We are concerned by the recent escalation at the border between Poland and Belarus," the NATO official said in a statement, remaining anonymous in line with the alliance's media practices.

"We call on Belarus to abide by international law. We have seen a surge of migrants trying to enter Allied territory via Belarus. NATO continues to closely monitor the situation, which is putting pressure on our Allies Lithuania, Latvia and Poland," the official said.

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