Chinese authorities battling a blaze aboard an Iranian oil tanker said Wednesday no major spill has been detected, but an explosion had forced firefighting vessels temporarily to suspend work.

One body has been found but 31 sailors from the Sanchi -- mainly Iranians -- remained missing four days after it collided with a freighter off Shanghai.

The tanker owner said it was hopeful that some or all of the crew members were in a safe part of the vessel and would be rescued.

Cleanup and rescue ships have faced toxic fumes, rain and windy conditions as they scrambled to find survivors and avoid a massive oil slick since Saturday's incident.

The Sanchi, carrying 136,000 tonnes of light crude oil, has been in flames since colliding with the CF Crystal, a Hong Kong-registered bulk freighter, 160 nautical miles east of Shanghai.

Experts had warned that a spill of the Panamanian-flagged 274-metre (899-foot) tanker's cargo could spell environmental catastrophe, as authorities said the ship could explode or sink.

But China's transport ministry said in a statement that as of 6:00 pm Tuesday, "no large-scale oil spills were found on the sea surface" where the search is being conducted around the stricken vessel, which continued to burn.

The oil from the tanker is condensate that is expected quickly to evaporate upon hitting the water, with "very little residue on the water's surface", the ministry said.

A simulation test also found that less than one percent of oil content would remain on the sea surface five hours after a condensate oil leak.

Thirteen vessels are still searching for missing crew members within 900 square nautical miles of the tanker, according to the ministry.

It said a Japanese vessel had joined the firefighting efforts and ships were attempting to use foam to extinguish the blaze, but the measures had "not had the anticipated effect".

The ships temporarily halted work Wednesday afternoon after they were forced back by an explosion, the ministry added.

Weather conditions are also unfavourable, with "overcast and rainy weather", strong winds and waves.

- 'Our priority is lives' -

Of the 30 Iranians and two Bangladeshis on the Sanchi's crew, only one body has so far been found.

"Our priority is the lives of the crew," said Mohsen Bahrami, a spokesman for the National Iranian Tanker Company (NITC) which operates the country's fleet of oil tankers.

"We still hope that at least some of them -- and I hope all -- are inside a protected part of the tanker," he said on Iranian state television.

"There is a good possibility that people are still alive," he added, while indicating that he had no solid information.

"We do not have the same priorities as the Chinese: they think first about the environment and prefer that the whole load burns, because it is a fishing area with a lot of fish", Bahrami said.

The oil tanker was on its way to South Korea when it collided with the CF Crystal, which was transporting grain to mainland China. The Crystal's 21 Chinese crew members were all rescued.

The Sanchi was transporting the oil to South Korea's Hanwha Total, Iran's petroleum ministry said. The ship and its cargo were insured.

This is the second accident in less than two years involving a tanker owned by the NITC.

In August 2016 an Iranian supertanker and a container ship collided in the Singapore Strait, causing damage to both vessels but no injuries or pollution.

Source: AFP