An intense fire at a Venezuelan refinery has spread to a third fuel tank, nearly three days after an explosion killed at least 41 people and injured more than 150.

Vice President Elias Jaua said in a message on Twitter that a third tank had  ignited at the Amuay refinery, which has been in flames since the blast on Saturday (local time).

Government officials had previously said they had the blaze contained, and the spread to another fuel tank was a setback to their plans to quickly restart the refinery. Oil Minister Rafael Ramirez told reporters the fire was still under control.

"There is no risk of a bigger event," Ramirez said, without specifying how much longer it might burn.

Officials have said a gas leak led to the blast, but investigators have yet to determine the precise causes. Prosecutor General Luisa Ortega said at a news conference that 151 people were injured, 33 of whom remain in hospitals.

A 9-year-old girl was missing in the area, Health Minister Eugenia Sader said on television.

Criticisms of the government's response to the gas leak emerged from local residents as well as oil experts. People in neighbourhoods next to the refinery said they had no official warning before the explosion hit, about 1am on Saturday.

"What bothers us is that there was no sign of an alarm. I would have liked for an alarm to have gone off or something," said Luis Suarez, a bank employee in the neighbourhood. "Many of us woke up thinking it was an earthquake."

The blast knocked down walls, shattered windows and left streets littered with rubble.

People who live next to the refinery said they smelled strong fumes coming from the refinery starting between 7pm and 8pm on Friday, hours before the blast. But many said they weren't worried because they had smelled such odours before.

Then, a cloud of gas ignited in an area with fuel storage tanks and exploded.
President Hugo Chavez visited the refinery on Sunday. In a televised conversation with the president, one state oil company official said workers had made their rounds after 9pm  and hadn't noticed anything unusual. The official said that at about midnight officials detected the gas leak and "went out to the street to block traffic".

"And later something happened that set (it) off," Chavez said. "A spark somewhere."

Energy analyst Jorge Pinon said the accounts of the hours leading up the explosion raise concerns.

"The fact that the gas leak went undetected for a number of hours and that there was no evacuation alarm (or) order indicates to me that there is a lack of safety related planning and behaviours throughout the complex, and most important in nearby communities," Pinon said.

"The key to refinery safety is not only equipment and maintenance but processes and behaviours," Pinon added, "not only within company employees but also contractors and surrounding communities."

US refineries have also had their share of serious accidents, most recently the destructive blaze at Chevron's refinery in Richmond, California.

Some experts say that US refineries have increasingly used more sensing systems to alert workers to gas leaks, and also have established safety protocols.

In the Houston area, for instance, "there are 10 or 11 different community groups that the various industries meet with frequently. They stay pretty well connected, with a set agenda," said Alex Cuclis, a research scientist at the Houston Advanced Research Center who used to be a refinery engineer.

"They have a phone number to call. And the industry can and occasionally does set off alarms to ‘shelter in place' and most who live in the communities know that means shut off air conditioners so that they aren't bringing in outside air," Cuclis said.

Amuay is among the world's largest refineries and is part of the Paraguana Refining Center, which also includes the adjacent Cardon refinery. Together, the refineries process about 900,000 barrels of crude per day and 200,000 barrels of gasoline.

The disaster occurred little more than a month before Venezuela's upcoming October 7 presidential election. Opposition candidate Henrique Capriles said the disaster shouldn't be politicised but repeated past criticisms about the number of accidents at the state-owned oil company.

"Accidents occur for a reason, and we Venezuelans are expecting there to be a conclusive response, a serious, responsible and transparent investigation, in order to see what the situation was," Capriles said at a news conference.

Capriles has previously been critical of problems in the oil industry.

"Look at how many events have occurred, how many accidents, how many workers have lost their lives," Capriles said. He criticised state oil company president Rafael Ramirez for what he called "political maneuvering," saying what's needed is a serious investigation

- AP