Mr Duterte made the comment during a national address on Friday as his country reported a new record 4,063 coronavirus infections, making it the worst-affected Southeast Asian nation for the second day running.
It is not the first time that the brash president, a lawyer by profession, has tried to offer his citizens bizarre and frankly hazardous advice about controlling the virus.
Last week, he said that people should “at the end of the day, hang [your mask] somewhere and spray it with Lysol [a popular disinfectant brand] if you can afford it”.
“For people who don’t [have Lysol], drench it in gasoline or diesel… just find some gasoline [and] dip your hand [with the mask] in it.”
After those comments, the president’s spokesman said it was only a “joke” and accused media sites who reported it seriously that “after four years of him as president, you still don’t know [him]”.
But Mr Duterte is now insisting the comments were not a joke. “What I said about alcohol is true,” he said. “If there is no available alcohol, especially if you’re poor, go to gasoline station and get some. That’s disinfectant.”
Of his earlier comments, he said: “They said, ‘Duterte’s insane.’ Stupid! If I’m insane, you should be the president, not me. I’m not joking. That is true. You only think I am joking.”
Besides the obvious fire risk, there is no evidence that gasoline can actually disinfect masks and prolonged exposure to it is harmful.
“Gasoline must not be used as a disinfectant,” the Integrated Chemists of the Philippines said in a Facebook post shortly after Mr Duterte’s address on Friday.
The Philippines has now recorded more than 93,000 coronavirus cases and over 2,000 deaths. On 13 July its daily death toll of 162 was a record for any country in Southeast Asia.
Mr Duterte said existing restrictions in the capital Manila and a few other central cities would be extended until mid-August. They include curbs on internal travel, movement restrictions for the elderly and children and limits on some business operations.
“My plea is to endure some more. Many have been infected,” Mr Duterte said.
The president downplayed the threat of the virus early on in the pandemic, rejecting calls in February to stop flights from China and saying that “even without the vaccines it will just die a natural death”.
The country later imposed one of the world’s toughest lockdowns, which took a toll on its economy but mostly kept the virus at bay. In May, polls suggested some 71 per cent of people felt Mr Duterte had handled the crisis well.
But that approval rating has crashed as cases rose fivefold once the lockdown was eased from 1 June, with the capital particularly badly affected. The greater metropolitan area of Manila, home to 13 million people, accounts for more than half of all cases.
Mr Duterte insisted on Friday that there was also good news, saying that China had promised the Philippines would be given priority in supplies should it make a breakthrough with a Covid-19 vaccine, and that the poorest would receive it first, followed by the middle classes and security forces.
“I promise you, by December, by the grace of God, we will be back to normal,” Mr Duterte said.