There was an explosion aboard the Iranian oil tanker that collided with a Chinese grain ship after rescue crews tried to use foam to extinguish the fire, China's Transport Ministry (MOT) said in a statement on Wednesday

The explosion happened at around 13:35 Beijing time on Wednesday

Emergency ships were forced to withdraw to safe distance after the explosion, the MOT said.

Just hours before the explosion, South Korea's Ministry of Oceans and Fisheries told Reuters the stricken Iranian oil tanker in the East China Sea could burn for as long as one month, as the blaze raged for a fourth day following a collision with a freight ship.

Dozens of rescue boats battled strong winds, high waves and poisonous fumes to locate 31 missing sailors and tame the ship fire, said a ministry official.

"We believe flames would last for two weeks or a month considering previous cases of oil tank accidents," said official Park Sung-Dong.

The tanker Sanchi, run by Iran's top oil shipping operator, National Iranian Tanker Co, collided on Saturday with the CF Crystal, carrying grain from the United States, about 300 km off China's coast near Shanghai.

It was carrying 136,000 tonnes of condensate, an ultra light crude that is highly flammable and to South Korea, equivalent to about 1 million barrels and worth about $60 million.

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