Dozens of people are missing and thousands of barrels of oil feared to have been released into the East China Sea following a collision between two vessels.
The collision between an oil tanker and a cargo ship occurred about 160 nautical miles (296 km) off the coast of Shanghai.
The Panama-registered vessel, carrying 136 000 tonnes of Iranian oil, caught fire after the crash.
The crew of the cargo ship has been accounted for, but China's Ministry of Transport said the missing people - 30 Iranians and two Bangladeshis - were crew of the oil tanker Sanchi.
Eight Chinese ships have been sent for the search and rescue operation, China's official Xinhua news agency said.
“After coordination by the China Maritime Search and Rescue Center, the ROK dispatched a coast guard ship and a fixed-wing aircraft to assist in the search and rescue,” Xinhua said.
The 274m long tanker had been sailing to Daesan, South Korea from Kharg Island, Iran, according to Reuters ship tracking data.
It was carrying a $60m (£44m) cargo, equivalent to slightly under a million barrels.
The CF Crystal cargo ship - registered in Hong Kong - was carrying 64 000 tons of grain from the US to Guangdong province in southern China, the BBC reported.