Russia’s Wagner Group might carry out “sabotage actions” and their threat should not be underestimated, said Poland’s Prime Minister Mateusz Morawiecki on Thursday, warning that the mercenary group’s provocations are an attempt to destabilize NATO.

Morawiecki and Lithuania’s President Gitanas Nausėda met at the Suwałki Gap to discuss the threat posed by the Wagner forces, some of whom have relocated to Belarus following the aborted mutiny in June against the Kremlin.

“Our borders have been stopping various hybrid attacks for years,” Morawiecki said. “Russia and Belarus are increasing their numerous provocations and intrigues in order to destabilize the border of NATO’s eastern flank.”

Nausėda echoed the sentiment, saying the presence of Wagner mercenaries in Belarus is a security risk for Lithuania, Poland and other NATO allies.

“We stay vigilant and prepared for any possible scenario,” Nausėda wrote on social media. Morawiecki said that the number of Wagner mercenaries in Belarus could exceed 4,000.

The Polish prime minister also thanked Lithuania for “military cooperation and for the joint promise that we will defend every piece of land of NATO countries.”

“Today, the borders of Poland and Lithuania are the borders of the free world that stops the pressure from the despotism from the East,” he said, about Russian President Vladimir Putin’s ongoing war on Ukraine.

Nausėda said that any closing of the border with Belarus is a decision that should be taken “in a coordinated way between Poland, Lithuania and Latvia,” national broadcaster LRT reported.

Some Wagner troops have moved to Belarus from Russia under a deal to end the group’s 24-hour rebellion against Moscow led by Wagner chief Yevgeny Prigozhin. The move immediately sparked tension with Belarusian neighbors, prompting Poland to re-station military units to the east of the country, closer to the frontier with Belarus.

Tensions escalated Tuesday when Poland moved troops to its border after accusing two Belarusian helicopters of breaching its airspace. Belarus denied the accusation, but Poland notified NATO and summoned Belarusian representatives to discuss the incident.