Poland plans to build a fence along its border with Belarus and deploy more soldiers there to stop migrants seeking to enter the European Union nation.

The government on Monday also offered to send humanitarian aid to a group stuck at the border for more than two weeks.

Poland and the three Baltic states — Latvia, Lithuania and Estonia — accuse Belarusian President Alexander Lukashenko of sending migrants — most apparently from Afghanistan and Iraq — across their borders, which are also the EU's external border, in what they call a “hybrid war.”

All four EU nations believe the surge in migrants is Minsk's revenge for supporting EU sanctions against the autocratic regime in Belarus.

“Using immigrants to destabilize neighboring countries constitutes a clear breach of the international law and qualifies as a hybrid attack against ... Latvia, Lithuania, Poland and thus against the entire European Union,” they said in a joint statement Monday, urging the United Nations to look into the situation.

In response to the migrants' arrival, the Polish government last week said it had deployed over 900 soldiers to the border with Belarus and was reinforcing the border with 150 kilometers (93 miles) of barbed wire. On Monday, Defense Minister Mariusz Blaszczak said more soldiers would be sent and that a fence 2.5 meters (8 feet) tall would be erected on the border.

The Polish government said last week that 2,100 migrants had tried or managed to enter Poland illegally from Belarus so far in August. Almost 800 of them got into Poland and have been placed in state-run centers.

Meanwhile, political tensions are growing in Poland over some 30 migrants who became stuck on the border with Belarus. A refugee rights group says the group includes people from Afghanistan and some who need medical attention. Poland insists they are on Belarusian territory, but has still faced criticism at home for not allowing the migrants to apply for asylum.

On Monday the Polish Foreign Ministry said it submitted a diplomatic note to Belarus offering to provide food and medicine as well as tents, beds, sleeping bags, blankets and pajamas.

Also Monday, Blaszczak, the defense minister, said he was sending a notification to prosecutors against Wladyslaw Frasyniuk, a prominent anti-communist dissident, for criticizing Polish soldiers deployed to the border. Frasyniuk had said Polish soldiers at the border were not behaving in a humane way, accusing them of acting like a “pack of dogs."

Deputy Foreign Minister Pawel Jablonski said the situation on the border was testing how the country would react to more serious acts of hybrid warfare.

“The statements and behavior of a significant number of Polish politicians, journalists and NGO activists show that a scenario in which a foreign country carrying out such an attack against Poland will receive support from allies in our country is very real,” Jablonski said on Twitter.

He said authorities should use this situation to “better prepare for similar threatening actions in the future."

In Warsaw about two dozen protesters chained themselves to a fence in front of the Border Guards headquarters and put barbed wire on its gates to protest the behavior of Polish authorities along the border.

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