Bunker fuel may now be leaking from the Iranian oil tanker that sank in the East China Sea last Sunday, China’s State Oceanic Administration (SOA) said, underlining fears for contamination from the world’s worst oil ship disaster in decades.

In a statement issued late on Thursday, the SOA said several previously unreported slicks were spotted by planes, vessels and satellites near the disaster site. The SOA said one, seen 2.5 kilometres (km) east of the wreck site around 0600 local time on Thursday, may indicate leakage of extremely toxic bunker fuel, the heavy oil used in ship engines.

It remain unclear how much bunker fuel was left aboard the tanker, the Sanchi, when it sank. Experts estimated it may have been carrying about 1,000 tonnes at the time it collided with the CF Crystal grain freighter.

Bunker fuel is noxious to marine organisms and difficult to remove from the sea once spilled, unlike the condensate fuel — an extremely light form of oil — that was being shipped by the Sanchi at the time of the collision.

The Sanchi was carrying 136,000 tonnes of condensate before the accident, most of which evaporated after the stricken shipped burned for days.

The SOA said it will continue carrying out monitoring and environmental impact assessment works.

Three slicks covering a combined area of 20.7 square km were spotted by satellite, with the largest in size 17.4 square km, the SOA said in its statement.

That combined area was 80.3 square km smaller than the total reported a day earlier. But water samples taken at four of the total 22 spill sites detected so far were found to exceed petroleum substance standards.