With around 90,000 troops taking part, the Steadfast Defender exercises mark a show of strength from NATO as countries on its eastern flank boost their defence spending in the face of Russia's invasion of Ukraine.

The Polish leg of the exercises, called Dragon 24, sees troops crossing the Vistula river before travelling around 300 km to the Suwalki gap, a strip of land nestled between the Russian exclave of Kaliningrad and Belarus.

"These are our joint exercises that strengthen the security of NATO's entire eastern flank. Poland is ready to implement our obligations arising from collective defence at any time, it is determined," Duda said during a joint speech with the president of Lithuania, Gitanas Nauseda.

Duda said the exercise showed the readiness to defend the territory of NATO, including Poland, Lithuania and the Baltic states.

"Lithuania can count on us that if there was such a need, Polish soldiers would stand shoulder to shoulder with Lithuanian forces to protect every inch of land from an enemy attack," he said.

Tuesday's exercises involved crossing the Vistula River with tanks and armoured vehicles from various NATO countries a point that is 340 meters wide.

"This is a serious test for the soldiers, but it shows their excellent preparation and our readiness to move forces from west to east in response to a possible attack on NATO forces in our part of Europe," Duda said.

Next week, Duda and Prime Minister Donald Tusk will go to Washington to mark 25th anniversary of Poland joining NATO, and discuss the situation in Ukraine, during a meeting with President Joe Biden.

(Reporting by Pawel Florkiewicz; Editing by Alison Williams)