The 32 missing crew members were all from the Iranian tanker Sanchi, carrying 136,000 tonnes of oil the time of the crash.
The Iranian oil tanker Sanchi burns after its collision with a freighter off China's east coast. The size of the oil slick caused by the accident also was not known.
By comparison, the Exxon Valdez was carrying 1.26 million barrels of crude oil when it spilled 260,000 barrels off Alaska in 1989.
The Panama-flagged oil tanker collided with the Hong Kong freighter on Saturday night, approximately 300 kilometres east of Shanghai.
Chinese officials dispatched several maritime police vessels, two rescue boats, and three professional cleaning ships to the area.
Chinese maritime authorities have sent eight ships for the search and rescue operation and South Korea has sent a plane and a 3,000-tonne coastguard ship to help. Rescue efforts are being hampered by fire in and around the tanker and by bad weather.
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The ministry said all 21 Chinese crew members were rescued from the damaged cargo vessel.
Reuters ship tracking data shows Sanchi was built in 2008 and was managed by the National Iranian Tanker Co (NITC).
An official in Iran's Oil Ministry, who spoke to the Associated Press on condition of anonymity, said 30 of the tanker's 32 crew members were Iranians. Norwegian ship insurer Skuld has confirmed it was the lead hull insurer for the tanker and the protection and indemnity (P&I) insurer for CF Crystal.
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.
This is the first major accident involving an Iranian oil tanker since global sanctions were lifted in January 2016.
CF Crystal, which was built in 2011, was due to arrive in China on January 10, according to Reuters ship tracking data.