A 15,000-ton cargo ship sank on Saturday while trying to berth in the Huangpu River near Nanpu Bridge in Shanghai.

Attempts will be made to salvage the Yin Chu this week but it will be a complicated operation, according to maritime officials.

"No one was injured in the accident and no oil polluted the river," said Zhou Zhengbao of the Shanghai Maritime Safety Administration.

"The work didn't begin right after the accident because we need some time to work out a salvage plan for such a big ship."

All holes in the ship were blocked soon after the accident to prevent pollution, Zhou said.

Two tugboats berthed beside the 150-meter former roll-on-roll-off Yin Chu to prevent it from floating to the middle of the river.

It had been turned into a dredge ship by Ruike Shipping Company and was being tested in the river when the accident occurred about 12:30pm.

The company believes the sinking was probably caused by an unclosed door. Water apparently entered from the door and the ship sank in two hours.

But company officials said the exact reason will not be pinpointed until salvage work is completed.