Beijing (CNN) Part of an oil tanker that has been ablaze for days off the coast of Shanghai exploded Wednesday, forcing rescue boats searching for 31 missing sailors to retreat, Chinese authorities said.

The Panama-registered tanker, Sanchi, was carrying 136,000 tons -- around 1 million barrels -- of oil from Iran to South Korea when it collided with the Hong Kong-registered CF Crystal freighter in the East China Sea on Saturday evening. Since then, the tanker has been ablaze and drifting in the waters between Shanghai and southern Japan.

The Japanese KOSHIKI vessel helps extinguish the flames on the SANCHI on Wednesday.

By Wednesday evening, the stricken ship had drifted 65 nautical miles southeast from the site of the collision, China's Ministry of Transport said. The front section of the SANCHI had exploded earlier Wednesday, at around 1.30 local time (12.30 a.m ET).

"Vessels at the scene have to stop putting out the fire and withdraw back to (a safe distance)," the statement said. "The fire extinguishing operation did not achieve the desired effect."

Thirty Iranian and two Bangladeshi sailors were onboard during the catastrophic collision. So far, no survivors from the tanker have been found and only one body has been recovered.

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