Two US surveillance aircraft flew over South Korea on Wednesday, an aviation tracker said, spurring speculation that the US may be strengthening its watch over North Korea after detecting unusual activity.

The US Navy's EP-3E was spotted in the skies over South Korea at 25,000 feet, Aircraft Spots tweeted.

An E-8C aircraft, or JSTARS, of the US Air Force also flew over the Korean Peninsula at 31,000 feet, it added.

The US surveillance aircraft have often been detected here in recent months, apparently to intensify monitoring of the bellicose regime.

Amid stalled denuclearization talks with the US, Pyongyang earlier this year warned of a "new strategic weapon" in the near future and a "shocking actual action."

Experts say the "strategic weapon" may mean an advanced type of its intercontinental ballistic missile (ICBM) or a submarine-launched ballistic missile (SLBM). (Yonhap)