Two US spy planes have flown over South Korea, an aviation tracker said Tuesday, on an apparent mission to monitor North Korea.

The US Air Force's E-8C and Navy's P-3C were spotted in skies above South Korea, Aircraft Spots tweeted without specifying the exact time of their operation.

The United States has deployed a series of reconnaissance aircraft here in recent months following the North's stepped-up military moves amid little progress in denuclearization negotiations with Washington.

The aviation tracker said Monday the US Air Force's RC-135W Rivet Joint was seen operating above the peninsula, and also said Wednesday its RC-135U Combat Sent was spotted en route to the East Sea.

North Korea fired what appeared to be surface-to-ship cruise missiles off its east coast last week, marking the fifth major weapons test the communist nation has conducted this year.

The deployment of such reconnaissance aircraft by the US is part of its regular operations, but it might have let some of them be spotted intentionally to send a message of pressure to the North, according to sources and experts.

An increase in surveillance activity could also suggest unusual moves may be under way in the communist nation, such as preparations for missile launches. (Yonhap)