An investigation team has been established to examine the oil pipeline explosion that occurred in northeast China's coastal city of Dalian Friday, local authorities said Sunday.

The investigations began Sunday morning but the cause of the accident is yet to be determined, said Sun Benqiang, deputy chief of the municipal work safety bureau.

A 0.9-meter-diameter oil pipeline exploded at 6 p.m. Friday near Dalian's Xingang Harbor, Liaoning Province, triggering an adjacent smaller pipeline to explode too.

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

The fire was mostly extinguished Saturday morning, 15 hours after the initial explosion.

The accident happened after a 300,000 tonne oil tanker discharged its load at the harbor. The tanker left the harbor safely.

"Whether CNPC or the tanker should be blamed for the accident is yet to be determined," said Sun.

The explosion and fire caused no casualties, a city government spokesman said. The scene of the accident is far from residential areas.

The spill "seriously polluted" 11-square-kilometers of sea and "slightly affected" 50-square-kilometers of sea, said Wu Guogong, deputy chief of the municipal environmental protection bureau.

CNPC said Sunday it would do its best to reduce the impact of the oil pipeline explosions.

Oil has stopped leaking into the sea as a valve has been closed. The oil that entered the sea has been fenced off and contained, CNPC said.

Monitoring of the air and sea environment has been stepped up in the affected areas, CNPC added.

The accident has aroused the attention of Chinese leaders President Hu Jintao and Premier Wen Jiabao. Vice Premier Zhang Dejiang rushed to the scene Friday night to direct operations.

Smoke from the explosions and fires at the scene is thinner than before but still irritates people's eyes and throats, said Xinhua reporters at the scene.

The pipelines linked on-land oil tanks and tankers.