A North Korean cargo ship suspected of smuggling coal out of the country reportedly sank in the Yellow Sea last month after colliding with a Chinese vessel.

Citing a source familiar with the matter on Thursday, Seoul-based Yonhap News reported that the North Korean cargo ship, which was sailing with its automatic identification system turned off, sank after colliding with a Chinese vessel in waters near a southeastern Chinese port in late February. 

Rescue operations led by Chinese authorities reportedly saved just a few North Korean crew members, and 15 to 20 others are believed dead. 

The Chinese ship reportedly sustained minor damage.

North Korea and China appear to have kept quiet about the accident since exports of North Korean coal are in violation of the UN Security Council sanctions.