Two US reconnaissance planes recently flew over Seoul and its surrounding area, an aviation tracker said Friday, on an apparent mission to intensify their surveillance of North Korea in the wake of its short-range missile firings earlier this month.

According to Aircraft Spots, an RC-135U Combat Sent and an RC-135W Rivet Joint were detected flying over Seoul and its surrounding Gyeonggi province on Thursday.

It is rare for two spy planes to fly simultaneously over Seoul.

The deployment came five days after a US Navy EP-3E signals reconnaissance aircraft flew over the South Korean capital.

The beefed-up reconnaissance on the Korean Peninsula is seen as intended to detect any signs of any additional military moves by Pyongyang after its missile firings earlier this month.

North Korea fired off short-range missiles on May 9, less than a week after it flew a barrage of projectiles into the East Sea.

This came as the North last carried out a major weapons test in November 2017.

The North called them part of its ordinary military drills to "inspect the ability of rapid reaction" of its defense units.

The US reconnaissance aircraft occasionally flew over areas near Seoul around the time when the North fired a string of ballistic missiles in 2017. Since the peace process began last year, however, such flights have usually taken place over the Yellow Sea, according to experts. (Yonhap)