Oil Minister Rafael Ramirez said a third tank ignited at the Amuay refinery, which has been in flames since Saturday's blast.
Government officials had previously said they had the blaze contained, and the spread to another tank was an apparent setback to their plans to quickly restart the refinery. While a thick column of smoke blew in the wind, Ramirez told reporters the fire was still contained.
"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 neighborhoods next to the refinery said they had no official warning before the explosion hit at about 1 a.m. 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 neighborhood. "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 7 p.m. and 8 p.m. Friday, hours before the blast, but many said they weren't worried because they had smelled such odors 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 9 p.m. 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."
The disaster occurred little more than a month before Venezuela's upcoming Oct. 7 presidential election. Opposition candidate Henrique Capriles said the disaster shouldn't be politicized, but he also strongly criticized a remark by Chavez, who had said "the show should continue, with our pain, with our sorrow, with our victims."
"It seems irresponsible, insensitive... to say 'the show should continue,'" Capriles told reporters in Caracas. The opposition leader also 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.
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 behaviors 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 behaviors," Pinon added, "not only within company employees but also contractors and surrounding communities."
U.S. 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 U.S. 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.
Associated Press writers Ian James and Fabiola Sanchez in Caracas contributed to this report.