Thick clouds of dark smoke could be seen billowing out of the Sanchi tanker, engulfing the vessel as rescue efforts were hampered by bad weather and fire on and around the ship, Mohammad Rastad, head of Iran's Ports and Maritime Organisation, told Iranian television.
The Sanchi collided with the CF Crystal about 160 nautical miles off the coast near Shanghai and the mouth of the Yangtze River Delta on Saturday evening, the Chinese Ministry of Transportation said in a statement.
The Panama-registered tanker was sailing from Iran to South Korea, carrying 136,000 tonnes of condensate, ultralight crude. That is equivalent to just less than 1 million barrels, worth about $60 million, based on global crude oil prices.
"Sanchi is floating and burning as of now," the ministry said. "There is an oil slick and we are pushing forward with rescue efforts."
State media CCTV showed pictures of the tanker ablaze and billowing plumes of thick dark smoke.
It had sent four rescue ships and three cleaning boats to the site by 9 AM (0100 GMT) on Sunday, it added.
South Korea also sent a ship and helicopter to help. A Korean Coast Guard official confirmed the tanker was still raging at 1 PM (0500 GMT). He declined to be identified as he was not authorised to speak to the media.
The tanker's 32 crew members were all Iranian nationals except for two Bangladeshi nationals, the Chinese transport ministry said.
"There is a wide perimeter of flames around the vessel because of the spillage and search and rescue efforts are being carried out with difficulty," Rastad said.
"Unfortunately, up to this moment, there is no news of the crew," he said.
CF Crystal's 21 crew members, all Chinese nationals, were rescued and the ship suffered "non-critical" damage, China's transport ministry said.
It was not immediately clear how much environmental damage had been caused or the volume of oil spilled into the sea.
About 63,000 tonnes of fuel oil leaked into the Atlantic, damaging beaches in France, Spain and Portugal and forcing the closure of Spain's richest fishing grounds.
Major Maritime Incident
The incident also marked the first major maritime incident involving an Iranian tanker since the lifting of international sanctions on Iran in January 2016.
There was a collision involving an NITC-operated supertanker in the Singapore Strait in August 2016, but there was no loss of life or pollution.
The Chinese government gave no details of the size of the spill. The Foreign Ministry said in a separate statement that the cause of the incident was under investigation.
Reuters ship tracking data shows Sanchi was built in 2008 and was managed by the National Iranian Tanker Co (NITC). Its registered owner is Bright Shipping Ltd.
It was due to arrive at Daesan in South Korea from Kharg Island in Iran on Sunday, according to Reuters ship tracking.
The Sanchi tanker, leased by Hanwha Total Petrochemical Co Ltd, had "valid foreign insurance", Iranian oil ministry spokesman Kasra Nouri told Iran's state television.
Hanwha Total was not immediately available for comment.
Sanchi collided with CF Crystal, registered in Hong Kong, which was carrying 64,000 tonnes of grain from the United States to China's southern province of Guangdong, the government said.
CF Crystal, which was built in 2011, was due to arrive in China on January 10, according to Reuters ship tracking data.