More than 30 crew members are missing after an oil tanker collided with a freighter off China's east coast, state media has reported.

An operation was under way to rescue the crew members, believed to be 30 Iranians and two Bangladeshis, according to the Xinhua news agency.

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

A photo of the accident scene broadcast on state television showed large amounts of black smoke billowing from the burning tanker.

The incident also 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.

"The tanker is still at sea and is still burning," the Transportation Ministry reported on Sunday. "There is oil on the surface of the water."

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 Panama-flagged oil tanker collided with the Hong Kong freighter CF Crystal on Saturday night, approximately 300km east of Shanghai.

The 21 crew members of the freighter were rescued and there was no major damage reported to that vessel. Chinese officials said they could not yet explain why the two ships had collided.