Police in Dalian, Liaoning province, have detained five people they say are responsible for damage to a crude-oil pipeline that triggered a massive oil leak and fire on Monday night.

The five were responsible for a drilling operation that damaged the pipeline near Dalian University in the city's Jinzhou New District, the Dalian emergency office said.

No casualties were sustained in the incident, but residents living nearby had to be evacuated and all the city's fire engines were mobilized, according to Ministry of Public Security's fire department, the nation's leading firefighting agency.

Firefighters extinguished the blaze by early Tuesday.

It was the sixth fire since 2010 involving the city's oil refining industry, which belongs to China National Petroleum Corp, the country's largest oil and natural gas producer and supplier.

A woman surnamed Piao, who was evacuated from her home during the incident, said,"Such accidents have happened each year in the city, and each time people were punished. But this doesn't prevent the accidents from occurring again."

Piao said she first heard about the incident when a friend phoned her before she left her home. She did not return until 2 am on Tuesday.

The company said on its website that it stopped the oil supply to the pipeline after discovering the leak before enacting an emergency response scheme.

Photographs online showed a huge amount of crude oil leaking into the city's sewer system. The authorities said the leak did not pollute tap water lines in the city.

However, the Dalian environmental protection bureau said it had monitored a level of volatile organic compound - organic chemicals that can cause respiratory diseases or even cancer - that was 0.79 times higher than the normal level.

The Liaoning Maritime Safety Administration said the incident only caused minor pollution to coastal waters in Dalian, as it was able to block most of the crude oil from entering the sea.

The city's emergency office said on Tuesday that gas and oil temperature levels in some areas have been declared safe, and residents have returned to their homes.

It said CNPC workers are still trying to recycle the leaked oil and a further investigation is under way.

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