The United States flew a surveillance aircraft over South Korea on Tuesday, an aviation tracker said, apparently continuing its mission to monitor North Korea amid concerns about possible provocations.

The US Air Force RC-135W Rivet Joint was spotted flying over the region at 31,000 feet, according to Aircraft Spots. The US flew the same type of spy plane on Monday and an E-8C, or JSTARS, the previous day. 

The US military has been intensifying its surveillance on North Korea as Pyongyang has warned of taking an unspecified new way if Washington fails to come up with a new proposal in their stalled denuclearization talks before year's end.

The series of flights appears to be aimed at monitoring the North's key missile launch sites or its transporter erector launcher vehicles amid speculation that the regime could conduct long-range rocket launches.

North Korea has been holding a key party meeting since Saturday, in which its leader Kim Jong-un has called for "diplomatic and military countermeasures" to protect the country's sovereignty and security.

Decisions from the plenary meeting will likely be reflected in a much-anticipated New Year's Day address Kim is expected to deliver Wednesday, amid growing signs that he could announce a major policy shift in its denuclearization talks with the US.

North Korea had said it is entirely up to Washington what "Christmas gift" it wants to get, spawning speculation a major provocation might be forthcoming. The holiday passed without a test, but Washington and Seoul remain on high alert.

US Secretary of State Mike Pompeo said Monday he hopes that North Korea will choose "a path of peace, not confrontation" as Washington is closely watching what leader Kim is to announce in his New Year's Day address. (Yonhap)