According to the Yonhap news agency, an RC-135W Rivet Joint reconnaissance plane of the US Air Force was spotted above the South Korean capital of Seoul at around 10 a.m. (01:00 GMT), with no callsign reported.
Citing unnamed sources and experts, the news agency said that the fact that the aircraft was let spotted could be aimed at putting pressure on North Korea.
On Saturday, North Korean leader Kim Jong-un held an expanded meeting with the country's top intelligence officials to discuss the enhancement of Pyongyang's "war deterrent" on the backdrop of perceived elevation of security risks in the peninsula.
Following a relative quiet in the inter-Korean relations during the coronavirus outbreak, tensions heated up in June as North Korea was angered by a campaign of South-based defectors flying anti-Pyongyang leaflets on balloons across the border. The North threatened the South with a state-level action and even blew up the joint communications office.