The EU has said Belarus is “attacking” the bloc by illegally forcing migrants to cross into countries like Latvia, Lithuania and Poland.
Tensions between the union and Belarus heightened after ruler Alexander Lukashenko cracked down on his opponents following a disputed election which he claims he won.
This year, more than 4,100 asylum seekers have crossed illegally from Belarus into Lithuania – 50 times more than the whole of 2020.
Most of the people were originally from Iraq, and are being housed in temporary camps across Lithuania.
Poland has moved an extra 1,000 troops to its border with Belarus to help the border guards already stationed there.
German Chancellor Angela Merkel and Estonian Prime MInister Kaja Kallas accused Mr Lukashenko of launching a “hybrid attack” on the EU.
Mr Kallas said: “This is no refugee crisis, but this is a hybrid attack on the European Union.”
The movement of migrants spiked after the contested election last year, which saw opposition leader Sviatlana Tsikhanouskaya flee to Lithuania.
Following crackdowns on protesters, the EU imposed trade sanctions on Belarus, with further action taken after a flight over the country was diverted by military planes and the journalist Roman Protasevich was arrested.
Many of the people crossing the Belarus border are believed to have landed in the country on commercial flights from Iraq – although these have stopped for the moment.
Lithuania’s border guard released footage which it claims shows Belarus riot police forcing people to cross the border.
In a statement, the current holder of the EU’s presidency – Slovenia – said: “This aggressive behaviour … is unacceptable and amounts to a direct attack aimed at destabilising and pressurising the EU.
“The European Union will need to further consider its response to these situations in order to increase its effectiveness and to deter any future attempts to instrumentalise illegal migration in this manner.”
They added that the bloc “recognise[s] the need to strengthen the entire external border of the European Union to prevent illegal border crossings in the future” – although no details were given.
David Sassoli, the president of the EU Parliament, said Mr Lukashenko was “exploiting these poor people, men and women.”
“I have seen these outrageous actions when officials push people across the border. It is both an issue of human rights, and also a question of protecting the border of the EU,” Sassoli said. “It is an organized activity of the Lukashenko regime.”
Ms Merkel said she would raise the issue with Russia in Moscow on Friday, which backs the Belarus regime.