Chinese authorities dispatched police vessels and three cleaning ships to the scene after the collision, which happened late Saturday. The South Korean coast guard also sent a ship and a plane to help search for the missing crew members — 30 Iranians and two Bangladeshis.

The Panama-registered tanker Sanchi was sailing from Iran to South Korea when it collided with the Hong Kong-registered freighter CF Crystal in the East China Sea, 257 kilometers (160 miles) off the coast of Shanghai, China's Ministry of Transport said.

All 21 crew members of the Crystal, which was carrying grain from the United States, were rescued, the ministry said. The Crystal's crew members were all Chinese nationals.

It wasn't immediately clear what caused the collision.

State-run China Central Television reported Sunday evening that the tanker was still floating and burning, and that oil was visible in the water.

It was not clear, however, whether the tanker was still spilling oil. The size of the oil slick caused by the accident also was not known.

Earlier Sunday, Chinese state media carried pictures of the tanker on fire with large plumes of smoke.

The Sanchi was carrying 136,000 metric tons (150,000 tons, or nearly 1 million barrels) of condensate, a type of ultra-light oil, according to Chinese authorities.

By comparison, the Exxon Valdez was carrying 1.26 million barrels of crude oil when it spilled 260,000 barrels into Prince William Sound off Alaska in 1989.

The Sanchi has operated under five different names since it was built in 2008, according the U.N.-run International Maritime Organization. The IMO listed its registered owner as Hong Kong-based Bright Shipping Ltd., on behalf of the National Iranian Tanker Co., a publicly traded company based in Tehran. The National Iranian Tanker Co. describes itself as operating the largest tanker fleet in the Middle East.

An official in Iran's Oil Ministry, who spoke to The Associated Press on condition of anonymity because he was not authorized to speak to reporters, said 30 of the tanker's 32 crew members were Iranians.

"We have no information on their fate," he said. "We cannot say all of them have died, because rescue teams are there and providing services."

The official said the tanker was owned by the National Iranian Tanker Co. and had been rented by a South Korean company, Hanwha Total Co. He said the tanker was on its way to South Korea.

Hanwa Total is a 50-50 partnership between the Seoul-based Hanwha Group and the French oil giant Total. Total did not immediately respond to a request for comment.

It's the second collision for a ship from the National Iranian Tanker Co. in less than a year and a half. In August 2016, one of its tankers collided with a Swiss container ship in the Singapore Strait, damaging both ships but causing no injuries or oil spill.