Belarusian president Alexander Lukashenko has warned that "further provocations are possible" in his country's growing stand-off with Poland on the EU's eastern frontier.
Thousands of people from Syria, Iraq and other countries are stranded without shelter in freezing temperatures, as Polish and Belarusian soldiers face each other down in a growing geopolitical crisis.
On Wednesday, Polish defence minister Mariusz Blaszczak confirmed "multiple" breaches" of its border with Belarus within the previous 24 hours, but said "all who managed to pass have been detained".
The EU and Nato accuse Belarus of attracting migrants from the Middle East and corralling them to its border region with Poland, in retaliation against European sanctions on Minsk over its treatment of political dissidents.
Polish prime minister Mateusz Morawiecki has accused Russian president Vladimir Putin of "masterminding" the crisis.
In response Mr Lukashenko said it was “bastard” European leaders who were using migrants to threaten Minsk and that “all provocations are possible”.
“If, God forbid, even the smallest mistake happened, that would immediately draw in Russia, the world’s greatest nuclear power,” he told Russian magazine Natsionalnaya Oborona.
Late on Tuesday night and early on Wednesday, Polish border police said two groups, of 80 and 200 people, mostly Kurds, made it through the border but were all returned to Belarus.
“Now they are attacking the frontier simultaneously in various smaller but still sizable groupings,” said Mariusz Blaszczak, Polish defence minister, promising to increase as needed the 15,000 soldiers and border guards on duty in the region.
On a visit to Warsaw, European Council president Charles Michel said the EU stood by Poland and would impose extra sanctions on Minsk as well as airlines involved in transporting migrants.
Though unable to convince Warsaw to accept assistance from Frontex, the EU border service, he promised to “keep the channel of dialogue open on this topic”.
Mr Morawiecki, the prime minister, called the stand-off an “orchestrated spectacle” and an unprecedented test of EU security and integrity.
"It's not just that violence is being used at our eastern border, against the sovereign Polish state – a spectacle is being staged there in order to disrupt our border, to foment chaos in Poland and the European Union. "
In a phone call with Moscow, Germany’s acting chancellor, Angela Merkel, told Vladimir Putin it was “unacceptable and inhuman” to instrumentalise migrants. She asked the Russian leader to intervene with Minsk.
Hours earlier, Russia sent two nuclear-ready Tupolev bombers to patrol Belarusian airspace in response, it said, to “Nato defence build-up on the border”.
After a surge in people-smuggling in the region, an estimated 4,000 people – including women and children – are stuck in freezing temperatures in makeshift camps in the forested border region. Poland has declared a state of emergency and has refused access to all media and human rights groups.
Humanitarian organisations are appealing to EU member states to demand that Poland grant access.
Kyle McNally, humanitarian affairs adviser for the aid agency Médecins Sans Frontières, met with Syrians in Belarus who say they were beaten by Polish and Lithuanian border guards.
“We hear rhetoric about ‘hybrid warfare’ but this is not a war and these are people, not weapons,” said Mr McNally. “The longer European authorities deliberately obstruct humanitarian assistance the more precarious their situation will become.”