Thirty-two people are missing after an oil tanker and a cargo ship collided off China's east coast.
The Sanchi tanker, carrying 136,000 tonnes of Iranian oil worth $60m (£44m), caught fire after the crash.
China's ministry of transport said the crew - 30 Iranians and two Bangladeshis - were missing from the Panama-registered vessel.
The 21-strong crew of the cargo ship had been rescued. "Sanchi is floating and burning," the ministry said.
"There is an oil slick and we are pushing forward with rescue efforts."
Poor weather and huge plumes of smoke rising from the tanker are making rescue attempts difficult, Mohammad Rastad, head of Iran's Ports and Maritime Organisation, told Iranian television.
It is the first major accident involving an Iranian oil tanker since international sanctions limiting its export were lifted in January 2016.
A collision between an Iranian supertanker in the Singapore Strait in August 2016 caused no loss of life or pollution.
Korean coast guard handout shows the fire on the Sanchi off China's east coast.
The tanker was on its way to South Korea
Saturday evening's collision happened about 160 nautical miles (296 km) off the coast of Shanghai.
Eight Chinese ships have been sent for the search-and-rescue operation, China's official Xinhua news agency said.
South Korea has also sent a coastguard ship and a helicopter to aid the relief effort.
The tanker had been sailing to Daesan in South Korea from Kharg Island in Iran, according to Reuters ship tracking data.
It was carrying a cargo equivalent to slightly under a million barrels.
The Hong Kong-registered cargo ship, CF Crystal, was carrying 64,000 tonnes of grain from the US to Guangdong province in southern China. Its rescued crew were all Chinese nationals, the country's transport ministry said.