Morawiecki and Nausėda held talks in the Polish city of Suwałki near Russia’s westernmost region of Kaliningrad, public broadcaster Polish Radio’s IAR news agency reported.
The Polish Prime Minister's Office said the talks focused on topics including the movement of Russia's Wagner Group forces in neighbouring Belarus.
It added that the Wagner Group's "presence in this area is a test by Russia and Belarus meant to check the response of Poland and our allies."
Russia, Belarus seeking to 'destabilise NATO's eastern flank'
The Polish prime minister told reporters that Russia and Belarus were "stepping up their numerous provocations in order to destabilise the border of NATO's eastern flank."
"Our borders have been blocking various hybrid attacks for years," Morawiecki said at a joint news conference with the Lithuanian president in Suwałki on Thursday.
He added, as quoted by his office: "We warn against provocations from Russia and Belarus. The Wagner Group may carry out sabotage actions, and anyone who plays down this threat may be responsible for further provocations and intrigues."
Ahead of the meeting, Morawiecki said in a video message that "cooperation between Poland and its ally Lithuania is essential in the face of threats and provocations from Russia and Belarus."
'We stay vigilant and prepared for any scenario': Lithuanian president
Lithuania's Nausėda hailed a "productive meeting" with Morawiecki on Thursday "at the Suwałki Corridor to discuss the security situation at our borders."
He said that "the presence of Wagner mercenaries in Belarus is an additional security risk factor for Lithuania and Poland and NATO allies."
"We stay vigilant and prepared for any possible scenario," he declared.
Earlier this week, the Polish government decided to move additional troops and combat helicopters to the Belarusian border after two Belarusian helicopters crossed into Polish airspace during an air exercise in the area, according to officials.
The Polish prime minister said at the end of July that Poland was guarding its border with Belarus "with full determination" against a threat from Wagner Group mercenaries stationed there.
Morawiecki said at the time that more than 100 Wagner fighters had moved to the Suwałki Corridor area on the Polish-Lithuanian border, which separates the Russian exclave of Kaliningrad on the Baltic Sea from Belarus.
Lithuania's Nausėda said on Monday that the presence of Wagner fighters near his country's border represented “a serious threat.”
He added that the Lithuanian army would support the border guards if necessary.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Thursday is day 526 of Russia’s war on Ukraine.
Source: IAR, PAP