China National Petroleum Corp. has suspended sales of fuel to North Korea over concerns the state-owned oil company won't get paid, as pressure mounts on Pyongyang to rein in its nuclear and missile programs, three sources told Reuters.

It's unclear how long the suspension will last. A prolonged cut would threaten critical supplies of fuel and force North Korea to find alternatives to its main supplier of diesel and gasoline, as scrutiny of China's close commercial ties with its increasingly isolated neighbor intensifies.

CNPC and the Ministry of Commerce did not respond to requests for comment. China's Foreign Ministry declined immediate comment. North Korea's Embassy in Beijing declined to comment.