According to Aircraft Spots, a RC-135S Cobra Ball plane was presumed to have carried out a surveillance mission over South Korea's East Sea after taking off from the Japanese territory of Okinawa, reports Yonhap News Agency.
Another RC-135S aircraft was spotted flying in the same route later.
On the same day, an E-8C plane or JSTARS was also seen flying over the peninsula at some 31,000 feet, the aviation tracker said.
The flights came after the US flew four spy planes at the same time over the Peninsula earlier this week in an unusual move to intensify its surveillance on North Korea amid concerns that the North could test-fire an intercontinental ballistic missile.
North Korea has threatened to take a "new way" if Washington fails to come up with a new proposal in denuclearization talks before the end of this year, hinting that it otherwise will end diplomacy and revert to provocative behaviour, said Yonhap News.
With the deadline drawing closer, the North earlier said that it was entirely up to Washington what "Christmas gift" it wants to get, spawning speculation that it might be preparing a major provocation during the holiday season this week.
The Seoul government said that it is closely monitoring any developments in North Korea in cooperation with the US and remains ready to address any possible situations.
Earlier on Thursday, Meari, a North Korean propaganda outlet, denounced the stepped-up surveillance activities by the US and South Korea on the communist state's military targets, calling them "provocative" moves.
( With inputs from IANS )