Poland has announced plans to build a new fence along its border with Belarus to stop migrants illegally entering the country.
The Polish Defence Ministry announced that more soldiers will also be deployed to protect the frontier.
EU countries have accused Belarusian President Alexander Lukashenko of deliberately facilitating the illegal migration of migrants.
The prime ministers of Estonia, Latvia, Lithuania, and Poland have urged the United Nations to take action against Belarus, accusing Lukashenko of carrying out a "hybrid war".
They say Minsk is encouraging migrants to enter the EU in revenge against sanctions on Belarus following Lukashenko's disputed re-election in 2020 and subsequent crackdown on dissent.
"Using immigrants to destabilise neighbouring countries constitutes a clear breach of the international law,'' the four nations said in a joint statement.
In response to the migrants' arrival, the Polish government last week said it had deployed over 900 soldiers to the border.
On Monday, defence minister Mariusz Blaszczak said more soldiers would be sent and a fence 2.5 metres tall would be erected along most of the 150-kilometre border, with construction due to begin next week.
"We are dealing with an attack on Poland, it is an attempt to trigger a migration crisis,'' Blaszczak said at a news conference.
The Polish government said last week that 2,100 migrants had tried to enter Poland illegally from Belarus so far in August, with around 800 placed in state-run detention centres.
Concern for those in No Man's Land
Political tensions are also growing in Poland over a group of around 30 migrants who are stuck on the border with Belarus.
A refugee support group says they are originally from Afghanistan and that some need medical attention.
The migrants -- which include three women and a 15-year-old girl -- has been stuck near the Polish village of Usnarz Górny for around two weeks, according to Fundacja Ocalenie (Salvation Foundation).
On Monday the Polish foreign ministry said it submitted a diplomatic note to Belarus offering to provide food and medicine for the group, as well as tents, beds, and blankets.
Poland insists the migrants are on Belarusian territory but has still faced criticism at home for not allowing them to apply for asylum.
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 behaviour of Polish authorities along the border.
Prime Minister Mateusz Morawiecki said last week that he sympathised with the migrants, but insisted they were "a tool in the hands of Lukashenko" and vowed that Poland would not succumb to "this type of blackmail".
Lithuania has also stood firm and said on Monday it would complete a 508-km fence along its border with Belarus by September next year.