There are no residents within 3km of the affected site, and little "marine farming", the report added.

Chinese media reports suggested oil had stopped spilling into the sea but the reports did not explicitly say it had been contained, saying that the clean-up would take 10 days.

The clean-up campaign came a day after two pipelines exploded at an oil storage depot belonging to China National Petroleum Corp (CNPC), the country's biggest oil company, near Dalian's Xingang Harbour in the northeast province of Liaoning, triggering a blaze that burned throughout the weekend.

Fire under control

The fire has been brought under control, but some pockets continued to burn on Monday as firefighters sprayed cooling foam on the facility in a bid to extinguish it fully, reports said.

Al Jazeera's Melissa Chan, reporting from the Chinese capital, Beijing, said officials say the damage from the two explosions is serious.

"Hundreds of firefighters have been battling the blaze which started late on Friday," she said.

"Authorities have not given any clues as to the cause of the accident, saying only that a formal investigation has been launched."

The blast happened when a Liberian-flagged tanker was off-loading oil, and CNPC said monitoring of the air and sea environment had been stepped up in the affected areas.

No deaths or injuries have been reported, but state television said oil had contaminated the ocean off the port city in Liaoning province.

The Xingang oil storage site, where the explosion happened, is home to one of the country's first government-held emergency crude stockpiles and a larger commercial crude reserve base built by PetroChina, the listed arm of state owned CNPC.

It is also a transfer spot for two nearby major refineries, Dalian Petrochemical Corp and WEPEC, both operated by PetroChina with a combined crude processing capacity of 600,000 bpd.