A US surveillance aircraft flew over South Korea on Tuesday, an aviation tracker said, on an apparent mission to monitor North Korea shortly after the communist nation fired what appeared to be cruise missiles off its east coast.

The US Navy's EP-3E was spotted in skies above South Korea at around 10 a.m. (Seoul time), Aircraft Spots tweeted.

Earlier in the day, North Korea fired surface-to-ship cruise missiles near its eastern coastal town of Munchon at around 7 a.m., according to the Joint Chiefs of Staff.

The firings came on the eve of the 108th birthday of Kim Il-sung, the North's late national founder and grandfather of current leader Kim Jong-un.

In North Korea, the late leader's birthday is celebrated as one of the country's biggest national holidays, and Pyongyang has often held massive military events to mark the day.

Tuesday's launches also came just one day ahead of South Korea's general elections.

Amid little progress in nuclear negotiations, the North has called for boosting self-defense and fired a series of projectiles believed to be short-range ballistic missiles.

The latest missiles launch marked the fifth major weapons test the North has conducted this year. The previous four missile launches were all conducted in March. (Yonhap)