At least three people worldwide are infected with untreatable "superbug" strains of gonorrhoea which they are likely to be spreading to others through sex, the World Health Organisation (WHO) says.
WHO experts said on Friday that studies show a "very serious situation" regarding the highly drug-resistant forms of the sexually-transmitted disease.
It was only a matter of time before last-resort gonorrhoea antibiotics would be of no use, the agency said.
"Gonorrhoea is a very smart bug," said Teodora Wi, a human reproduction specialist at the Geneva-based UN health agency.
"Every time you introduce a new type of antibiotic to treat it, this bug develops resistance to it."
The WHO estimates 78 million people each year get gonorrhoea, an STD that can infect the genitals, rectum and throat.
The infection, which in many cases has no symptoms on its own, can lead to pelvic inflammatory disease, ectopic pregnancy and infertility, as well as increasing the risk of getting HIV.
Ms Wi said there were three documented cases - one each in Japan, France and Spain - of patients with strains of gonorrhoea against which no known antibiotic is effective.
"These are cases that can infect others. It can be transmitted," she said.
"And these cases may just be the tip of the iceberg, since systems to diagnose and report untreatable infections are lacking in lower-income countries where gonorrhoea is actually more common."
In a study, the WHO's programme for monitoring trends in drug-resistant gonorrhoea found that from 2009 to 2014 there was widespread resistance to the first-line medicine ciprofloxacin, increasing resistance to another antibiotic drugs called azithromycin, and the emergence of resistance to last-resort treatments known as extended-spectrum cephalosporins (ESCs).
In most countries, it said, ESCs are now the only single antibiotics that remain effective for treating gonorrhoea, but resistance to them has been reported in 50 countries.