A senior Polish official called for the “complete isolation” of Belarus as Warsaw deployed more troops to the border in response to what it called an airspace violation by Belarusian helicopters.

The move marks a sharp escalation in tension between Poland and its eastern neighbor after the Russian mercenaries from the Wagner group that stage an aborted mutiny decamped to Belarus. Polish and Baltic authorities have increasingly warned of a mounting security threat, calling for measures to punish Belarusian President Alexander Lukashenko.

“I am convinced that in cooperation with Lithuania and Latvia, we should work toward the complete isolation of Belarus in the future,” Deputy Interior Minister Maciej Wasik told public television Wednesday. “Lukashenko reacts only to force.”

Poland’s Foreign Ministry late Tuesday summoned Belarus’s ambassador and demanded an explanation for the violation of its airspace “immediately and in detail.” Belarus’s Defense Ministry denied any its Mi-24 and Mi-8 helicopters crossed over Polish territory, according to the state-owned Belta news agency.

Poland’s head of the National Security Bureau, Jacek Siewiera, said the country has prepared a variety of scenarios on possible provocations that could be carried out by the Wagner group mercenaries.

“Underestimating the activities of Wagner group would be a mistake,” Siewiera told Wprost newspaper.

Lithuania’s President Gitanas Nauseda also warned this week that the private military company could conduct “various provocations” across the border. US National Security Council spokesman John Kirby said Tuesday the US was unaware of specific threats posed by Wagner to Poland or any other NATO member.

But the issue has emerged as a political topic in Poland months ahead of an election. Polish Prime Minister Mateusz Morawiecki warned on Saturday that some of the fighters may attempt to infiltrate his country alongside illegal migrants, evoking Belarus’s decision in 2021 to send migrants — mostly from the Middle East — into the European Union across the Polish and Baltic borders.

Poland’s border protection agency said the Wagner presence in Belarus had heightened the security risk. The authority cited the gathering of larger groups throwing stones and firing blank bullets at border personnel. Hundreds of vehicles have also been damaged, customs official said.