Poland on Monday accused Russia and Belarus of orchestrating a wave of illegal immigration at its land border, a day after four migrants were found dead at its Belarusian frontier.
Thousands have been trying to cross from Belarus into EU members Latvia, Lithuania and Poland in recent weeks.
The EU suspects the influx of people mostly from the Middle East is being orchestrated by Belarusian President Alexander Lukashenko in retaliation for sanctions on his regime.
"We're dealing with a mass organised, well-directed action from Minsk and Moscow," Polish Prime Minister Mateusz Morawiecki told reporters in Warsaw, adding that up to 7,000 migrants had been spotted on the border since early August.
He said nobody believed Lukashenko was acting alone, adding that the Belarusian leader and his Russian allies were working with "great determination" to transport "tens of thousands" from the Middle East and Africa.
Those migrants are being used to apply the "pressure of illegal immigration to the EU's external border", he added.
'Irresponsible and inhuman'
Four people were found dead on Sunday on the Belarus-Polish border according to officials from both countries, a week after Warsaw imposed a state of emergency because of the migrant influx.
Eight exhausted migrants were also found stuck in marshy terrain elsewhere along the border, Polish border guards said. Seven of them required hospitalisation.
Belarus opposition leader Svetlana Tikhanovskaya said on Monday that Minsk was using the migrants in "revenge against Lithuania, Latvia and Poland for supporting independent democratic forces, the movement for change in Belarus".
Tikhanovskaya fled to Lithuania from Belarus last year after claiming victory against Lukashenko in a disputed presidential election which the opposition and Western powers say was rigged in his favour
"Migrants are hostage to this situation, it must end," Tikhanovskaya told guests of an economic forum in the southern Polish city of Katowice, calling the behaviour of Lukashenko's regime "irresponsible and inhuman".
Earlier this month, Poland imposed a 30-day state of emergency banning non-residents including media from the border area, the first time the country has used such a measure since the fall of communism in 1989.
It has also sent thousands of soldiers to the border and started building a barbed-wire fence.
In early August, Belarus said it had discovered a dead Iraqi man near its border with Lithuania, claiming he was murdered.
Western governments have slapped several sets of sanctions on Belarusian leaders over a crackdown on dissent that began when protests erupted across the country following a disputed election last year.