Donald Trump on Friday attempted to clean up a remark he made the previous night about the possibility of injecting disinfectant to treat coronavirus by saying he was merely being "sarcastic," a claim undermined by video of the comment.

"I was asking a question sarcastically to reporters ... to see what would happen," the president said Friday as he signed another coronavirus relief bill into law, contending he was asking his medical advisers to examine the impact sunlight has on the virus, not disinfectants, after a health official had described a study suggesting ultraviolet rays quickly kill Covid-19 cells. (A transcript of his Thursday press briefing, however, suggests otherwise.)

The president set off the latest controversy of his term when he appeared to suggest people could inject disinfectants into their bodies to terminate Covid-19 cells. The remark was almost immediately rejected by doctors on social media, and later by his own public health advisers, including those who were with him in the White House briefing room when he made the jaw-dropping comments.

"And then I see the disinfectant, where it knocks it out in a minute, one minute. And is there a way we can do something like that by injection inside or almost a cleaning, because, you see, it gets in the lungs and it does a tremendous number," Mr Trump said on Thursday.

He then instructed his top public health officials – including coronavirus task force member Deborah Birx, a physician who sat statuesque nearby as the president suggested Lysol or 409 injections as Covid-19 treatments – to "check that."

Mr Trump, who often flashes his sense of humour in public – made all of those remarks with a serious tone. His facial expression also was serious, not featuring the playful grin he displays when he is joking or expressing sarcasm.

The president's disinfectant idea appeared to have been triggered by an acting Department of Homeland Security official mentioning the use of those products on surfaces to kill the virus.

"We are also testing disinfectants, readily available. We have tested bleach, we have tested isopropyl alcohol on the virus specifically in saliva or respiratory fluids. I can tell you that bleach will kill the virus in five minutes, isopropyl alcohol will kill the virus in 30 seconds – and that is with no manipulation no rubbing, just spring it on and leaving it go," William Bryan, the acting DHS official, said a few minutes before Mr Trump's remark.

"You rub it and it goes away even faster," Mr Bryan added. "We are also looking at other disinfectants, specifically looking at the COVID-19 virus in saliva."

The incident prompted White House advisers to try to clean up the president's mess in Friday morning television appearances and in an official statement.

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"President Trump has repeatedly said that Americans should consult with medical doctors regarding coronavirus treatment, a point that he emphasised again during yesterday's briefing," new White House Press Secretary Kayleigh McEnany said in a Friday morning statement. "Leave it to the media to irresponsibly take President Trump out of context and run with negative headlines."