Chinese netizens and a state press report on Sunday questioned safety standards at oil firms after a fire erupted at a plant owned by state-run CNPC -- the industry's fourth incident in six weeks.

The country is still grappling with a huge oil spill off its eastern coast that has now contaminated an area around six times the size of Singapore, and another separate slick and fire have also tarnished the industry's image.

In the latest incident, a leak from a piece of oil refining equipment caught fire Saturday at a plant owned by state-run China National Petroleum Corp (CNPC), the country's biggest oil producer.

The company said in a statement Sunday the fire in the northeastern port city of Dalian had been put out, there were no casualties and tests showed the surrounding sea had not been polluted.

The fire came exactly a year after two pipelines exploded at an oil storage depot also owned by CNPC in the same city, triggering a devastating spill.

The government estimated about 1,500 tonnes of oil poured into the Yellow Sea, but Greenpeace said up to 60 times more may have escaped.

The state-run Beijing News on Sunday quoted anonymous people at the CNPC refinery as saying the workshop where the fire broke out had just been serviced.

It added the plant where this weekend's incident happened was close to the depot where the spill happened last year.

"Some observers are pointing out that the frequent incidents to have hit oil firms recently may not be unexpected, and loopholes in the firms' safety and production management need to be seriously examined," the report said.

Netizens -- already incensed about the oil spill off the east coast which was kept secret by authorities for several weeks before being made public this month -- also expressed anger at the latest embarrassing mishap.

"What is this? No doubt the fire erupted to test whether those who inspected the workshop were lazy...," one online user on popular web portal Netease.com said.

CNOOC, the state-owned oil giant accused of covering up the big spill, this month also had to clean up another small slick after a breakdown at one of its rigs.

In a separate incident, a CNOOC refinery in the southern province of Guangdong, located near the Daya Bay nuclear power plant, caught fire earlier this month.