YAN'AN, Shaanxi, May 15 -- The death toll in a coal mine accident in northwest China's Shaanxi Province has risen to nine, and efforts to rescue four trapped workers is continuing, local authorities said Thursday.

As of 7:40 p.m., rescuers had found the bodies of seven of the 11 trapped workers, bringing the death toll to nine.

Rescuers said survival chances of the other trapped miners were slim.

The accident occurred around 11 a.m. Wednesday at a coal mine under construction by China National Coal Group Corp in Yulin City .

An initial investigation showed that the accident happened at a shaft when the cement pipe fell off.

Thirty-seven workers were working under the ground, and 26 were lifted out. The injured were sent to local hospitals, and two of them died after treatment failed.

Currently, 16 workers were still receiving treatment at hospital, with six of them seriously injured.

A further investigation into the accident is under way.