Evacuation orders remain in place for an area covering tens of thousands of people in the Garden Grove suburb of Los Angeles.
California Governor Gavin Newsom declared a state of emergency for Orange County and his office pleaded for residents to follow evacuation orders.
Craig Covey, division chief of the Orange County Fire Authority, said crews had gone back into the danger zone in Garden Grove overnight after drone readings on Friday suggested water sprayed on the tanks was helping stabilise the situation.
But those drone readings measured the outside of the vessel, not the chemical inside, Covey said in a video update posted on social media on Saturday morning.
When crews reached the tank's gauge, they found the internal temperature was 32C, up from 25C when responders had pulled back.
The temperature was increasing by about one degree an hour, he said.
"That's the bad news," Covey said.
Officials have warned since Friday that the tank, which contains methyl methacrylate, a flammable chemical used in plastics and manufacturing, could rupture and spill up to 26,500 litres of toxic material or explode and endanger nearby tanks.
Firefighters were exploring whether a heavy flow of cooling water might slow the curing process inside the tank enough to reduce pressure and prevent an explosion.
"Letting this thing just fail and blow up is unacceptable to us," Covey said.
"Our goal is to find something and not allow that to happen."
The incident began on Thursday at the GKN Aerospace facility in Garden Grove, a city of about 172,000 people roughly 50km south of Los Angeles.
The site specialises in the manufacturing and testing of windows and canopies for commercial and military aircraft according to its website.
GKN said it is working with "all relevant experts" to solve the problem.
"We sincerely apologize for the significant disruption to the many local residents and businesses who have had to be evacuated," a GKN spokesperson said in a statement on Saturday.
Officials expanded evacuation orders on Friday after the risk of explosion increased.
About 15 per cent of people in the zone covered by the evacuation order are refusing to leave, Garden Grove Police Chief Amir El-Farra said, according to the Orange County Register.
Officials have opened evacuation shelters in Garden Grove and the nearby cities of Anaheim and Cypress.
Health officials have said they were concerned that vapour from the chemical could cause severe respiratory problems with prolonged exposure. Air-quality monitors had not detected vapour as of the latest health update cited by officials.
Covey said crews were also preparing for a possible spill by looking for ways to dike, dam and divert the liquid into a holding area at the commercial site, rather than allow it to reach storm drains, river channels or the ocean.
with AP
