Oil pipelines and equipment in Dalian, Liaoning Province, lie damaged after an explosion on Friday night. [China Daily]

Workers rushed to contain an oil spill covering up to 50 square kilometers from seeping further into the Yellow Sea on Sunday, two days after an explosion at an oil storage port in Dalian, Liaoning Province.

An oil pipeline exploded at 6:00 PM on Friday near Dalian's Xingang Harbor, triggering another blast at a smaller pipe nearby, Xinhua News Agency reported.

Both pipelines, owned by China National Petroleum Corp (CNPC), caught fire. Firefighters put out the blaze at the larger pipeline at around midnight. But at least five subsequent explosions fueled the fire at the smaller pipeline.

More than 2,000 firefighters fought the blaze and most of the fire was put out on Saturday morning, 15 hours after the initial explosion, Xinhua reported.

The local fire department was still cooling down affected oil tanks on Sunday to prevent any further risk of fire.

No deaths were reported in the fires.

The first blast occurred on Friday evening when a Liberian oil tanker was offloading oil, Chen Zhigang, an officer with the frontier station in Xingang Harbor, told China Daily.

An inspection team was formed on Sunday morning to investigate the blast but the cause of the accident has not yet been determined, said Sun Benqiang, deputy chief of the municipal work safety bureau.

President Hu Jintao and Premier Wen Jiabao issued instructions on firefighting work. Vice Premier Zhang Dejiang rushed to the scene on Friday night to direct operations.

After the fires were extinguished, workers began using oil skimmers and dispersants to contain the oil slick from spreading beyond the port into the Yellow Sea.

By Sunday evening, about 7,000 meters of floating booms had been set up and at least 20 oil skimmers were working to clean the spill, the Liaoning provincial maritime safety bureau said.

The affected region is about 100 sq km offshore, bureau officials said.

Eleven square kilometers of the ocean in the area are "relatively polluted", Wu Guogong, deputy director of Dalian's environmental protection bureau, told China Daily.

CNPC vowed on Sunday to "do its best" to reduce the impact of the explosion. Oil has stopped leaking into the sea as a valve has been closed, it said.

Oil that spilled into the sea has been fenced off and contained, CNPC said.

More than 20 air surveillance points and 10 offshore surveillance points are also monitoring the situation, Wu said.

There are no residents within 3 km of the affected site and there is "very limited" marine farming in the area, Wu said.

Still, nearby fishermen told Hong Kong-based Phoenix TV that they are deeply concerned about how the incident will affect their businesses.

About 600 families live within 4 km from the affected site. Dalian's downtown areas were also overshadowed by smog on Saturday. The hydrocarbon density was high within a 1.2 km radius from the site of the fire, local environment authorities said.

"Till now, the air quality has not exceeded the national warning standard," Wu said, adding that the burning of the crude oil will give out more than 40 kinds of noxious vapor.

Feng Lianyong, a professor with China University of Petroleum, said that, compared with the recent BP oil spill in the Gulf of Mexico, the incident in Dalian is "much more controllable".

Based on media reports, Feng said improper operation during oil uploading of a vessel might have caused the explosions. The blasts will only affect the harbor rather than extensive maritime areas and the reach can be seen and controlled, Feng said.