Firefighters warned on Saturday that a tank of toxic chemicals in California is heating up, adding to fears of a catastrophic explosion that has already forced tens of thousands of Californians to evacuate.

About 40,000 residents were ordered to leave their homes in the Garden Grove area of Orange county, southeast of Los Angeles, on Friday after the tank began to leak, sending fumes over a heavily populated area.

The tank contains 26,000 litres (7,000 gallons) of methyl methacrylate, a volatile and flammable liquid used to make plastics, with firefighters warning the situation was serious.

Orange county Fire Authority Incident Commander Craig Covey said on Saturday morning that an emergency team had ventured into the area overnight, seeking to neutralise the “explosive potential” posed by an additional 56,781-litre tank nearby should the 26,497-litre tank blow up, and were then able to view the temperature gauge on the 26,497-litre tank.

“Unfortunately, I do have to report that the temperature was 90 degrees Fahrenheit (32 degrees Celsius). Yesterday morning, it was 77 degrees when we backed out. It’s been averaging about a degree an hour increasing, so that’s the bad news,” he said in a short video posted on social media.

He said firefighters are seeking ways to cool the tank. Aerial footage filmed by local television stations on Friday showed jets of water being sprayed at the tank, which has a capacity of 128,704 litres.