Air traffic trackers said the US Air Force RC-135W Rivet Joint reconnaissance aircraft was spotted in the skies over Incheon, west of Seoul, and neighboring Gangwon and Gyeonggi provinces as well as the east and west coasts of South Korea on January 22.

The US reconnaissance aircraft was dispatched to South Korea three days after North Korea said it had tested its underwater nuclear strike weapon system, the Haeil-5-23, in the East Sea. Pyongyang said the move was in response to the latest joint naval drills by South Korea, the US and Japan last week.

The US deployment of advanced reconnaissance aircraft is part of routine intelligence-gathering operations, but revealing the flight paths could be intended to send a message of pressure to North Korea, sources said.

This is the third flight of the US reconnaissance aircraft this year after two previous flights on January 4 and January 17.