Some 32 crew members are missing after an oil tanker collided with a freighter off China’s east coast sparking a massive fire and a potential ecological disaster.

An operation was under way to rescue the crew members, believed to be 30 Iranians and two Bangladeshis, according to the China’s state-run Xinhua news agency.

There was no way to tell if there were any survivors.

The Panama-registeredvessel, named Sanchi, was listing heavily to one side and was threatening to capsize. Much of the 274-metre ship appeared to be on fire.

Images on television showed black smoke reaching up towards the sky.

The Panama-registered 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 kilometres 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 was not immediately clear what caused the collision.

“Sanchi is floating and burning,” the ministry said.

Poor weather and huge plumes of smoke rising from the tanker are making rescue attempts difficult, Mohammad Rastad, head of Iran’s Ports and Maritime Organisation, told Iranian television.

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.

A photo of the accident scene broadcast on China state television showed billows of black smoke rising from the burning tanker.

The incident has the potential to turn into an ecological disaster, after tens of thousands of tons of oil either ignited or poured into the water.

It was reportedly carrying 136,000 tons of oil from Iran to South Korea.

“There is an oil slick and we are pushing forward with rescue efforts,” the Transport Ministry said.

It was unclear how far the slick might spread.

China sent eight ships to help with rescue operations and to try to contain the slick. South Korea also sent a ship and a helicopter.

The South Korean coast guard also sent a ship and a plane to help search for the missing crew members

– with AAP