Moscow's Ukrainian invasion has spooked the region's small Baltic states -- including Lithuania -- as well as Poland, which neighbours Ukraine to the west. There are fears they could be next.

"The president of the Republic of Lithuania and I decided to come to this site and demonstrate that this place is safe", Polish President Andrzej Duda told reporters.

"It is safe because of what you can observe here with us today: the quiet, but absolutely vigilant, daily service of Polish, Lithuanian and NATO troops," he added in the Polish border city of Szypliszki.

The "Suwalki Gap", which gets its name from the area's main city, runs the length of the Polish-Lithuanian border and is sandwiched between Russia's heavily militarised Kaliningrad exclave and Belarus.

Military strategists warn it is the weakest link on NATO's eastern flank as its capture would amputate the alliance's three Baltic members and so shatter its credibility.

"We all know perfectly well that this is a vulnerable spot where the aggressor's eyes could be directed first," Lithuanian President Gitanas Nauseda told reporters.

But he recalled that NATO allies had decided to bolster the security of the Western defence alliance's eastern flank at their latest summit in Madrid.

"I am very glad that NATO seized this historic opportunity to pay attention to emerging risks and to respond to them very quickly and decisively," Nauseda said, flanked by armoured fighting vehicles and soldiers.

The heads of state visited with NATO troops on both sides of the border.