Poland is closing a border crossing with Belarus - it says migrants have tried to force their way in.
Video on Monday showed hundreds of migrants arriving at the Poland-Belarus border.
Humanitarian groups say they face freezing weather, and a lack of food and medicine.
The European Union has accused Belarus of encouraging thousands of migrants from the Middle East and Africa to cross into EU countries.
They claim it's a kind of hybrid warfare as revenge for western sanctions on President Alexander Lukashenko's government.
This follows Lukashenko's violent crackdown on protests after a disputed election last year.
Polish authorities have accused Belarus on Monday of trying to spark a major confrontation and are mobilising soldiers.
Lukashenko's government has repeatedly denied manufacturing a migrant crisis, instead blaming the West for the crossings and treatment of migrants.
On Monday Poland said it had withstood the first attempts by migrants to force their way across the border.
Authorities say they will close the crossing at Kuznica on Tuesday morning. It is one of three crossings along the Belarus border.
They said at least seven dead migrants have been found on Poland's side of the border, with unverified reports of more deaths in Belarus.
Neighbouring Lithuania also said it might introduce a state of emergency on its border with Belarus.
Humanitarian groups accuse Poland 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.