NATO surveillance planes were due to arrive in Romania on Tuesday to bolster the military alliance’s eastern flank and help monitor Russian military activity.

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.

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

The aircraft deploying to Romania belong to a fleet of 14 NATO surveillance aircraft usually based in Germany. Around 180 military personnel will be deployed in support of the planes.

“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.

“Our AWACS can detect aircraft hundreds of kilometers away, making them a key capability for NATO’s deterrence and defense posture.”