Two oil residue tanks exploded on Sunday in Dalian, Northeast China's Liaoning Province, leaving two critically injured and another two missing, local officials said.

The two blasts occurred at around 2:30 pm within one minute of each other at a PetroChina refinery in the Ganjingzi district of the city, China Central Television quoted a maritime officer surnamed Liu as saying on Sunday.

The Xinhua News Agency reported that the fire was extinguished at 4 pm.

The sound of explosion could be heard from far away. The site was surrounded by heavy smoke and black dust, and the ground covered in oil spots.

The tanks, used for storing tar, were under maintenance in the No.939 workshop when the explosion happened, said Liu, adding that the cause of the blast is still uncertain.

A South Korean freighter that was berthed in nearby waters was diverted soon after the blast, according to Liu.

The two injured workers, who were rushed to hospital, were reported by Xinhua to be in critical condition.

"Basically, the blast hasn't affected people's lives, since the facility is located near the coast and there are few residential communities nearby," said Liu, claiming real-time monitoring indicated no air or water pollution.

However, two local residents reached by the Global Times said they saw the heavy smoke and smelt something pungent.

"The black smoke poured toward our home and I kept the windows firmly closed," Wang Chun, a resident who lives just a few blocks away, told the Global Times, adding that she saw many people were strolling downstairs when the accident happened, but none of them was scared and everything seemed to be normal.

Around 100 fire trucks and over 300 firefighters were mobilized for the rescue, and Li Wancai, the city mayor, arrived on site to take charge of the operation.

Since July 2010, four fires have occurred in this branch of PetroChina.

Jiang Fan, a former general manager of the branch, who had served for nearly 10 years, was removed from the post in August 2011, and 34 officials responsible for the fire were given punishment in accordance with administrative or Party discipline.