A burning Iranian oil tanker that had drifted into Japan's exclusive economic zone has sunk over a week after it collided with another vessel, Chinese state television has said.

A Tehran official said even before news of the sinking that there was no hope of saving some 30 missing crewmen.

But Chinese officials played down fears of a major environmental disaster.

The Sanchi, carrying 136,000 tonnes of light crude oil from Iran, had been in flames since colliding with the CF Crystal, a Hong Kong-registered bulk freighter, on 6 January.

Around midday local time, the ship "suddenly ignited", with the entire vessel burning fiercely and a pall of smoke around 800-1,000 metres high, China's transport ministry said, releasing dramatic pictures showing the entire vessel obscured by thick black smoke.

The ship later sank, the official news agency Xinhua cited the State Oceanic Administration as saying.

"There is no hope of finding survivors among the members of the crew," Mohammad Rastad, spokesman for the Iranian rescue team dispatched to Shanghai, told Iran's state broadcaster in Tehran before the tanker went down.

Mr Rastad said information from members of the Crystal crew suggested all the personnel on the Sanchi were killed in the first hour of the accident "due to the explosion and the release of gas".

"Despite our efforts, it has not been possible to extinguish the fire and recover the bodies due to repeated explosions and gas leaks," he said.

The Sanchi, which was headed to South Korea to deliver its cargo, had a crew of 30-32 Iranians and two Bangladeshis. Only three bodies have so far been recovered.

Yesterday, Chinese rescuers also recovered the tanker's "black box", the transport ministry said without specifying exactly what had been retrieved.