Poland has stepped up security on its eastern border over concerns about a large group of migrants trying to enter the country from Belarus.
Video showed hundreds of people near a barbed-wire border fence, which some have tried to force their way through.
The Polish government had called a crisis meeting and put 12,000 troops on duty at the border.
Poland has accused Belarus of pushing the migrants towards the border, describing it as hostile activity.
Poland, Lithuania and Latvia say there's been a surge in the number of people trying to enter their countries illegally from Belarus in recent months. Many of them have come from the Middle East and Asia.
The European Union has accused Belarus's authoritarian President Alexander Lukashenko of facilitating the influx in retaliation against sanctions.
Poland, which has been criticised for pushing back migrants and refugees at the border, has responded to the large number of people by building a razor-wire fence.
Conditions for migrants on the border are hostile to the point of being deadly, and fears have been raised for their safety in the region's sub-zero winter.
As they are summarily expelled from Poland and Belarus refuses to allow them back in, people are finding themselves stranded and freezing in Poland's forests. Several have died of hypothermia.
On Monday, the Nato alliance said it was worried about "escalation" on the border with Poland and was ready to "maintain safety and security in the region".
Meanwhile in Lithuania, the government there moved troops to its border with Belarus to prepare for a possible influx of migrants and was considering declaring a state of emergency.
https://www.bbc.com/news/world-europe-59206685
