Andrzej Duda, President of Poland.

On Monday, Polish President Andrzej Duda said that once Finland and Sweden join NATO, the Baltic Sea will be under their full control.

"The Baltic Sea is essentially poised to become NATO's internal basin. Two very powerful nations with a long and strong military tradition will become part of NATO soon, extending the Russia-NATO border by 1,400 kilometers [870 miles]," Duda said during a national security council meeting.

Recently, both NATO and Russia have been keeping a close eye on the Baltic Sea, where Moscow has recently been conducting naval drills and exercises.

In May, Russia withdrew from the Council of Baltic Sea States, stressing that the withdrawal will not affect Russia's presence in the area.

In April, NATO Secretary-General Jens Stoltenberg said the alliance may increase its presence around Sweden and the Baltic Sea to "protect the country from Russian meddling during a potential membership application."

"We must remember that from the moment Sweden potentially applies, and NATO says that it wants Sweden to join, there is a very strong NATO obligation to guarantee Sweden's security", Stoltenberg told Swedish public television SVT.

While NATO, in June, conducted military drills coinciding with the decision to increase troop presence in Europe to 300,000, Russia has also been conducting military drills in the sea.