“Our borders have stopped various hybrid attacks for years," said Poland's PM Mateusz Morawiecki.
Polish and Lithuanian leaders held an urgent meeting on Thursday, warning they are bracing for possible provocations from Moscow and Minsk.
The meeting came two days after two Belarusian helicopters violated Polish air space, in what was viewed as an antagonistic move by the Kremlin's closest ally.
Both nations on NATO's eastern flank have beefed up border security following the arrival of thousands of mercenaries from the Russian Wagner group following their aborted mutiny in June.
Dr Stephen Hall, lecturer of Russian politics at the University of Bath, told Euronews on Monday Wagner could stage an attack to sever the Baltics from NATO, though he questioned if such a "suicidal" step would be taken, owing to immense geopolitical risks.
“Russia and Belarus are increasing the pressure on the borders," Polish Prime Minister Mateusz Morawiecki said at a news conference with Lithuanian President Gitanas Nauseda. "We must be aware that the number of these provocations will grow."
Belarus denied its helicopters entered Poland, while on Tuesday the country's Ministry of Defence called Warsaw's accusations an attempt "to justify once again" the increase in forces... near the Belarusian border".
Polish and Lithuanian leaders met in Suwalki, a town in the Suwalki Gap, a sparsely populated stretch of land running 65 km along the Polish-Lithuanian border.
The land corridor links the Baltics - comprising Lithuania, Latvia and Estonia - with the rest of the NATO alliance, and separates Belarus and the Russian enclave Kaliningrad.
Western military analysts worry that Russia could seize the area and isolate the Baltics, jeopardising NATO's ability to defend them.
"Russia's primary objective has always been to show NATO is just a paper tiger," Hall told Euronews. "By cutting off the Baltics they could highlight the alliance won't come to the aid of its members".
"This would be a major cataclysm for NATO," he continued, adding if the US-led military alliance did not come to the Baltics' aid in the event of an attack - as it is obligated to do - NATO would be "completely destroyed".
Lithuanian President Nauseda said he believes the Suwalki Gap remains vulnerable, even though Sweden — located across the Baltic Sea — is on track to join NATO.
“Some people say the agreement reached at the NATO summit on Sweden’s future membership is changing the geopolitical situation, and the strategic importance of the Suwalki Corridor is diminishing. I certainly do not agree with this view and I believe that the Suwalki Corridor remains a potential target of provocation by both Russia and Belarus,” he said.
Finland's entry into the US-led military alliance has changed the strategic makeup of the area, meaning the Baltics now have a defensive bulwark in the event of a Russian attack.
Around 100 Wagner troops have recently arrived in Belarus near the Suwałki Gap, prompting Polish PM Morawiecki to claim on Saturday it was "undoubtedly" a step towards a hybrid attack.
Others have tried to put ice on these fears. Hall told Euronews on Monday that Wagner's military strength is not sufficient to attack the area and pointed out any direct assault could prompt a military retaliation from NATO.
Countries on the alliance's eastern frontier have felt under pressure for a couple of years.
Large numbers of migrants from the Middle East and Africa have been channelled to the borders of Poland, Lithuania and Latvia by Belarus, which the EU accuses of waging “hybrid warfare” aimed at creating instability in the West.
Warsaw and Vilnius have been accused of violently pushing migrants back into Belarus in a situation likened to ping pong. These pushbacks are illegal under international law and experts have claimed treating migrants as a threat prevents society from looking at the situation as a humanitarian issue.
“Our borders have stopped various hybrid attacks for years," Polish PM Morawiecki said. "Russia and Belarus are increasing their numerous provocations and intrigues in order to destabilise the border of NATO’s eastern flank.”
“Today, the Polish borders and the border of Lithuania are the borders of the free world, which are stopping the pressure from the despotism from the east,” he added.