Hundreds of migrants camped out near the Belarus border with Poland on Tuesday in freezing overnight temperatures as the Polish Prime Minister visited the border and officials warned tension could increase in coming days.
Poland has accused Belarus of trying to spark a major confrontation by encouraging the migrants to cross into Poland and the European Union.
Video clips have shown hundreds of migrants walking towards the Polish border near Kuznica village and some trying to breach a fence using spades and other tools.
Footage published by the Polish police on Tuesday showed migrants' tents and campfires on the Belarus side of the barbed wire fence.
Poland's Border Guard told Reuters that about 800 people were camped out on the Belarusian side of the fence, part of a group of up to 4,000 migrants there and in nearby forests.
A spokesman for Poland's special services said estimates showed there could be up to 12,000 migrants in Belarus.
Polish authorities shut an official border crossing with Belarus at 0600 GMT on Tuesday near where thousands of migrants tried to push through the day before.
Poland said it had deployed additional soldiers, border guards and police, while neighbouring Lithuania said it might introduce a state of emergency on its border with Belarus.
Polish police said on Twitter on Tuesday that the night was calm, although a rock was thrown at a police car, after the Monday confrontation.
A Polish official said tension could increase in coming days and additional international help could be accepted if that were the case.
Deputy Foreign Minister Pawel Jablonski, speaking to private Radio Zet, restated that for now, Poland did not need additional help from the EU border guard Frontex.
European Union ambassadors at the United Nations are expected to meet to discuss the tension on Tuesday, the PAP news agency reported Poland's U.N. ambassador as saying.
The Belarusian state news agency Belta quoted the interior minister to say that no migrants had broken the law.