BEIJING (Reuters) - Rescue boats resumed their search in choppy waters of the East China Sea on Thursday for any survivors from the stricken Iranian oil tanker that collided with a freight ship and burst into flames at the weekend, the Chinese government said.

The ship remains at risk of exploding and sinking due to spilled oil as the fire raged for a fifth day, the Ministry of Transportation said in a statement on Thursday.

Rescue crews were forced to retreat on Wednesday after a blast on the tanker.

“Spilled oil covering the ship body and its surrounding water is still burning, which leaves Sanchi at risk of exploding and sinking,” said the MOT in the statement.

The ministry has sent 12 rescue ships to search for the 31 remaining tanker crew members and to extinguish the fire. One body suspected to be from the tanker crew was retrieved from the water this week.

“Contaminated air from fire and bad weather have increased difficulties of rescue work,” the ministry said.

By 5 p.m. (0900 GMT) on Thursday, tanker was still on fire and search area had expanded to more than 1,000 square miles.

The tanker, run by Iran’s top oil shipping operator National Iranian Tanker Co, hit the CF Crystal vessel that was carrying U.S. grain on Saturday.

The cause of the collision and damage to the tanker were not clear. CF Crystal is under investigation after arriving at Zhoushan port on Wednesday. Sailors from CF Crystal were rescued by a passing Chinese fishing boat on the night of the incident.