Polish border guards say some migrants trying to enter from Belarus have been found to have Russian visas, deepening Poland's suspicion that Moscow is behind an ongoing border stand-off.
Police report daily attempts to breach the Poland-Belarus border, an external border of the European Union, saying some migrants had flown in from Russia or travelled from the country by land.
The border was virtually unknown as a migration route until last year, but has come under sustained pressure for months in what the EU describes as a deliberate ploy by Belarus.
Poland suspects Russian involvement because of the close ties between Belarusian President Alexander Lukashenko and the Kremlin. Belarus and Russia are both under EU sanctions.
Polish government spokesman Stanislaw Zaryn said the latest accusations by border guards were "yet another trace showing the active participation of the Russian regime in this operation".
"For many days, the Border Guard has indicated that foreigners storming the Polish border come to Belarus from Russia," he said. ,
Among those who tried to cross the border in recent days included people from Egypt, Ethiopia, Iraq, Turkey and the Central African Republic, border police said.
Belarus is suspected of ferrying people to the border to cause a migration crisis and retaliate against EU sanctions linked to domestic repression by Mr Lukashenko.
The EU imposed further sanctions on five banks and 22 people and restricted trade with Belarus because of the country's involvement in the Russian war in Ukraine.
Poland, as well as Belarus's other EU neighbours, Lithuania and Latvia, have tightened security and built new border fences to limit the number of arrivals from Belarus.
The contrast between their tough line on the Belarusian border, and the much warmer welcome given to Ukrainian refugees since Russia invaded in February, has been widely noted and criticised.
Amnesty International on Tuesday criticised Latvia for proposing to extend a state of emergency on the border with Belarus until November.
"As Latvia welcomed more than 34,000 refugees fleeing Ukraine, migrants and refugees at the Belarus border ― mainly from Iraq and Afghanistan ― including children, were left to fend for themselves in freezing temperatures in the forest," it said.
