The European Union and NATO accuse Belarus of encouraging migrants from the Middle East and Africa to cross into EU countries via Belarus as a form of "hybrid warfare" in revenge for sanctions on Minsk.
Belarus's allegedly tactical use of refugees and migrants to pressure the EU is "unacceptable", NATO has said, warning about an "escalation" on the border with Poland.
An alliance official on Monday termed Belarus's use of migrants "a hybrid attack", meaning a combined military-political operation.
Polish authorities also accused Belarus of trying to spark a major confrontation on Monday as footage on social media showed hundreds of migrants walking from Belarus towards the Polish border.
Poland mobilised additional soldiers as did neighbouring EU member Lithuania.
READ MORE: Polish lawmakers give go-ahead to build Belarus border wall
The development appeared to signal an escalation of a crisis that has being going on for months in which Belarus has encouraged refugees and migrants to illegally enter the European Union, at first through Lithuania and Latvia and now primarily through Poland.
The European Union said it hoped that Poland, an EU member state, would finally accept help from Frontex, the border agency, a step Poland's ruling nationalists have so far refused.
"NATO stands ready to further assist our allies, and maintain safety and security in the region," NATO said.
READ MORE: Poland weighs declaring emergency to keep migrants out
A manufactured crisis?
Belarusian President Alexander Lukashenko's government has repeatedly denied manufacturing a migrant crisis, blaming the West for the crossings and treatment of migrants.
The Belarusian state border committee confirmed that many refugees were moving towards the Polish border, and said Warsaw was taking an "inhumane attitude".
The EU, the United States and Britain imposed sanctions on Belarus after Lukashenko unleashed a violent crackdown on mass protests following a disputed election last year.
In Brussels, spokesman for the European Commission, Adalbert Jahnz, noted that “several hundred people” were gathered near the border, an imprecise figure that he said came from Frontex.
He said, “This is a continuation of the desperate attempt by the (Belarusian President Alexander) Lukashenko regime to use people as pawns to destabilize the European Union and of course the values that we stand for.”
The blood of refugees
Charities say migrants face gruelling conditions trying to cross the border from Belarus in freezing weather with a lack of food and medical attention.
Polish authorities said seven migrants have been found dead on Poland's side of the border, with reports of more deaths in Belarus.
Humanitarian groups accuse Poland's ruling nationalists of violating the international right to asylum by pushing migrants back into Belarus instead of accepting their applications for protection. Poland says its actions are legal.
READ MORE: Poland 'unlawfully' pushed back Afghans on Belarus border