HONG KONG — An oil spill from an Iranian tanker that sank in the East China Sea is rapidly spreading, officials said Tuesday, alarming environmentalists about the threat to sea and bird life in the waterway.

The tanker, the Sanchi, was carrying 136,000 tons of highly flammable fuel oil when it crashed into a freighter on Jan. 6. On Sunday, the Sanchi sank after a huge blast sent up a great plume of black smoke and set the surface of the water on fire, China Central Television said.

The bodies of three crew members have been recovered, and the remaining 29 were presumed dead, the Iranian government said. Thirty Iranians and two Bangladeshis were believed to have died.

The oil slicks from the sunken tanker were growing in size, China’s State Oceanic Administration said Tuesday. There are now two huge slicks covering 52 square miles, compared with just four square miles the previous day. Strong winds were pushing the spill toward Japan, away from China, and it was now less than 200 miles from Naha, Japan.