The Polish Ministry of Defence has released heat camera footage showing people gathering in makeshift migrant camps on the border between Poland and Belarus.
The migrants – who are mainly young men – are enduring freezing conditions in the hopes of reaching the EU.
The EU has accused Belarus of luring migrants to the border to destabilise the blocs eastern border, in retaliation for sanctions imposed on Minsk.
Russia, Belarus’s main ally, has rejected these accusations.
The BBC’s Nick Beake has travelled to a forest where at least nine migrants are known to have died of hypothermia.