A NATO surveillance plane arrived in Romania on Tuesday to bolster the military alliance's eastern flank and help monitor Russian military activity.

The plane, the first to be deployed, landed at an air base near Bucharest and two more are expected to land later in the day and on Wednesday. They are due to stay for several weeks.

NATO announced last week it would deploy the Airborne Warning and Control System (AWACS) surveillance planes to Bucharest, where they will start reconnaissance flights solely over NATO territory.

"In the context of Russia's illegal war against Ukraine, NATO will monitor military activity on the eastern flank with the help of AWACS aircraft," Romanian Defence Minister Angel Tilvar said on Facebook.

NATO has boosted its air presence in eastern Europe and the Baltics since Russia invaded Ukraine in February last year, using fighter jets, surveillance planes and tankers.

The aircraft deployed to Romania belong to a fleet of 14 NATO surveillance aircraft usually based in Germany. They can detect other aircraft hundreds of kilometers away, NATO said.

"As Russia’s illegal war in Ukraine continues to threaten peace and security in Europe, there must be no doubt about NATO’s resolve to protect and defend every inch of Allied territory," NATO spokesperson Oana Lungescu said in a Jan. 12 statement on the deployment.