The spill from a sunken Iranian tanker off China's east coast has spawned four oil slicks as authorities prepared to send robots to the wreckage to assess the environmental damage.

The Sanchi, which was carrying 136,000 tonnes of light crude oil from Iran, sank in a ball of flames in the East China Sea on Sunday, a week after colliding with Hong Kong-registered bulk freighter the CF Crystal.

The bodies of only three out of 30 Iranian and two Bangladeshi crew members have been found.

The State Oceanic Administration of China said late Wednesday that it was monitoring four slicks with a total area of almost 101 square kilometres (39 square miles), roughly the same size as Paris.

The office is attempting to "control the spread of the oil spill and is carrying out work to estimate its impact on the marine ecological environment", it said on its website.

The type of condensate oil carried by the Sanchi does not form a traditional surface slick when spilt, but is nonetheless highly toxic to marine life and much harder to separate from water. The cargo amounted to nearly one million barrels (bbl) of oil.

Japan's coast guard said Thursday a patrol ship and plane examined the oil spill and found that it was spreading 30 kilometres east and eight kilometres northwest from the wreck, with a maximum width of 800 metres.

"The swathe of oil slick is being diffused and disappearing," the coast guard said in a statement. "Chinese and other patrol ships continue to look for missing (crew) members and carrying out oil removal missions."

On Tuesday, China had reported two slicks measuring about 69 square kilometres and an additional 40-square-kilometre area of "scattered" oil.

The transport ministry said late Wednesday the vessel lay at a depth of around 115 metres (50 feet) and that robots would be deployed to explore the shipwreck.

- Sea life at risk -

The area where the ship went down is an important spawning ground for species like the swordtip squid and wintering ground for species like the yellow croaker fish and blue crab, among many others, according to Greenpeace.

It is also on the migratory pathway of numerous marine mammals, such as humpback and gray whales.

In addition to the light crude oil, the Sanchi also carried a fuel tank able to accommodate some 1,000 tonnes of heavy diesel.

If all of the Sanchi's cargo spills into the sea, it would be the biggest oil slick from a ship in decades.

By comparison, in the sixth-worst spill since the 1960s, the Odyssey dumped 132,000 tonnes some 700 nautical miles off Canada's Nova Scotia in 1988, according to figures from the International Tanker Owners Pollution Federation website.

Richard Steiner, an Alaska-based oil spill consultant, called on the Chinese government to conduct a survey of the sunken vessel as soon as possible, citing concerns about the possibility of continued leakage.

"It is critical to know if any condensate remains on board," he said in an email, adding that instead of evaporating it could create a "more concentrated toxic underwater plume."

"If the entire 1 million bbl cargo is released (which seems likely at this point), this will be the largest condensate spill in history."