(Reuters) – A tanker carrying Iranian oil and run by the country’s top oil shipping firm was ablaze and spewing cargo into the East China Sea on Sunday after colliding with a Chinese freight ship, leaving the tanker’s 32 crew members missing, the Chinese government said.

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, run by the country’s top oil shipping operator, 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 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.

Norwegian ship insurer Skuld confirmed it was the lead hull insurer for the tanker and the protection and indemnity insurer for CF Crystal.

“We are working very closely with the relevant authorities to establish the facts surrounding the collision,” it said in a statement.

The Panama-registered tanker was sailing from Iran to South Korea, carrying 136,000 tonnes of condensate, an ultra light crude. That is equivalent to just under 1 million barrels, worth about $60 million, based on global crude oil prices.

“Sanchi is floating and burning as of now,” the Chinese ministry said. “There is an oil slick and we are pushing forward with rescue efforts.”

It had sent four rescue ships and three cleaning boats to the on Sunday, the ministry added.

Chinese state media CCTV showed pictures of the tanker ablaze and billowing plumes of thick dark smoke.

South Korea also dispatched a ship and helicopter to help.

The tanker’s 32 crew members are 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,” Iran’s Mr. 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.

Shipping experts said the incident could potentially disrupt shipping in and around Shanghai, one of the world’s largest and busiest ports.

It was not immediately clear how much environmental damage had been caused or the volume of oil spilled into the sea.

The last major oil tanker disaster was the sinking of the Prestige off Spain in November 2002, which was carrying 77,000 tonnes of heavy fuel oil and caused one of Europe’s worst environmental catastrophes.

About 63,000 tonnes of its cargo leaked into the Atlantic, damaging beaches in France, Spain and Portugal and forcing the closure of Spain's richest fishing grounds.

That was almost twice the size of the Exxon Valdez spill, which ravaged Alaska's coast in 1989.

The incident also marked the first major maritime incident involving an Iranian tanker since the lifting of international sanctions against 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. 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.

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 Chinese government said.

CF Crystal, which was built in 2011, was due to arrive in China on Jan. 10, according to Reuters ship tracking data.