Thousands of tons of oil are at risk of pouring into the sea off the coast of China after a tanker collided with a freighter.
Iranian vessel Sanchi struck Hong Kong-registered CF Crystal about 160 miles (257km) from Shanghai, leaving its 32 crew missing.
Sanchi was carrying 150,000 tons, or nearly 1 million barrels of condensate, a type of ultra-light oil, according to Chinese authorities.
The tanker is now on fire and is spilling its cargo into the sea, authorities have said.
It is the latest of dozens of shipping accidents in a region that has recently been dubbed the new Bermuda Triangle, because of the number of ships that founder there.
Thirty Iranians and two Bangladeshis were on board the Sanchi when it was involved in the collision.
South Korea’s coast guard has sent a ship and an airplane to assist the search effort.
All 21 Chinese crew members of the Crystal, which was carrying grain from the US, have been rescued.
The size of the oil slick spilling from the Sanchi is not currently known.
Asked about the 30 Iranians, an unnamed official in Iran’s Oil Ministry told the AP: “We have no information on their fate. We cannot say all of them have died, because rescue teams are there and providing services.”
The official said the tanker had been rented by a South Korean company and was on its way to South Korea.
Last year, German newspaper Die Welt carried out research which found the South and East China Seas and the seas around Indochina, Indonesia, the Malay Peninsula, the Korean Peninsula and Japan were the most dangerous in the world.
Some 33 ships were lost in 2016, according to the paper.
The risks presented by the region’s seas led Die Welt to headline the article “The new Bermuda Triangle lies in front of China”.
The Bermuda Triangle was a mythical area of the Atlantic Ocean between the Caribbean and Bermuda where dozens of ships and planes inexplicably sank or went missing in the 19th and 20th centuries.
Several US Navy ships have also ran into difficulty in the seas between Singapore and Japan, with the worst crash tearing a hole in the USS John S McCain and leaving 10 sailors missing.
Analysts say the region is so dangerous for ships because of the sheer amount of cargo being transported through its waters.