Chinese authorities have launched a clean-up operation to tackle 50 sq km slick following an explosion of two crude oil pipelines in Dalian.

Hundreds of firefighters worked for more than 15 hours to extinguish the blaze that started late on Friday when a pipe transporting crude oil from a ship to a storage tank blew up, causing a second pipeline nearby to explode.

There were no casualties, but state television said oil had contaminated the ocean off the port city in Liaoning Province.

State oil company PetroChina, which operates two major refineries in the area has set up a contingency plan to cope with one week's closure of the main oil port that receives foreign crude vessels regularly and is also a main export point for gasoline and diesel.

Workers are using skimmers and dispersants to break up the oil slick and stop it from spreading. So far about 7,000 metres of floating booms have been set up and at least 20 oil skimmers are working to clean the spill.

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.

Reports say the blast happened when a Liberian-flagged tanker was off-loading oil. The cause of the blast is under investigation, and CNPC, the parent of PetroChina, said monitoring of the air and sea environment had been stepped up in the affected areas.