A US reconnaissance plane flew over central South Korea on Tuesday, an aviation tracker showed, as North Korea appeared to be gearing up for a ruling party congress.

The US Air Force's RC-135W Rivet Joint was spotted flying over the southern coast and then the central regions Tuesday morning, according to the tracker, No callsign.

The aircraft, which specializes in detecting telemetry signals before missile launches, took off from Kadena Air Base in Okinawa, Japan.

The RC-135W Rivet Joint also analyzes trajectories of warheads.

The last time it flew over the Korean Peninsula was on Dec. 18.

This time it appeared to have flown at a lower altitude.

"The US is paying close attention to North Korea's movements, including preparations for a (possible) military parade for the 8th Congress of the Workers' Party of Korea in January," a South Korean military official said on condition of anonymity.

"The US military will likely conduct more frequent surveillance flights around the date of the congress," he added.

38 North, a US website monitoring North Korea, has said satellite imagery from Saturday showed thousands of people "apparently practicing for an upcoming event" at Pyongyang's Kim Il-sung Square.

North Korea is set to hold its first party congress in January in more than four years, drawing attention to whether it will unveil new policies on the United States and South Korea amid stalled denuclearization talks. (Yonhap)