US President Barack Obama has cancelled a meeting with controversial Philippine President Rodrigo Duterte, who had earlier called him a "son of a whore".
Mr Duterte was responding to the US president's promise to raise the issue of drug-related extra-judicial killings in the Philippines at their meeting.
The Philippine leader, known for his colourful language, has insulted prominent figures before, but this time it has had diplomatic consequences.
He has now said he regrets the remark.
"While the immediate cause was my strong comments to certain press questions that elicited concern and distress, we also regret that it came across as a personal attack on the US president," a statement by his office said.
In the past, President Duterte has called Pope Francis the "son of a whore", US Secretary of State John Kerry "crazy" and recently referred to the US ambassador to the Philippines a "gay son of a whore".
Both he and President Obama are in Laos for the Association of Southeast Asian Nations (Asean) summit.
Duterte's apology: Analysis by Karishma Vaswani in Laos
Mr Duterte has been forced to apologise for offensive comments before, but this is the first time he has had to confront the reality of his outlandish behaviour on the international stage
It is the president's first overseas trip - an opportunity that many leaders would have used to cement ties with neighbouring countries and superpowers like China and the US.
Instead Mr Duterte has spent the morning dampening down the controversy he created.
At the heart of this is the fact that Mr Duterte isn't used to being told what to do; and that he likes to display machismo and bravado, which plays well to his domestic audience.
But when he sits down for serious discussions with his Asean counterparts over the next couple of days, they'll be looking for Asian discretion and subtlety, not diplomacy Duterte-style.
How the row escalated
Mr Obama, who flew to Laos after attending the G20 meeting in Hangzhou, China, had been set to raise concerns about human rights abuses in the Philippines.
But speaking in Manila on Monday before he left for Laos, Mr Duterte bristled at the suggestion, saying the Philippines "has long ceased to be a colony".
"Putang ina, I will swear at you in that forum," he then said, using a Tagalog phrase for "son of a whore" or "son of a bitch".
Mr Obama initially appeared to play down the insult, calling his Philippine counterpart a "colourful character" and saying he had asked his aides to work out if this is "a time where we can have some constructive, productive conversations".
His aides later cancelled the talks.
Mr Obama's last scheduled trip to Asia as president has not been without incident: he was also caught up in a protocol row with hosts China over his arrival in Hangzhou.
In his comments on Monday, President Duterte pledged to continue with his anti-drugs campaign that has led to the killing of 2,400 suspected drug dealers and users in the Philippines since he took office in June.
"Many will die, plenty will be killed until the last pusher is out of the streets... until the [last] drug manufacturer is killed we will continue," he said.
The UN has repeatedly condemned Mr Duterte's policies as a violation of human rights. In August, two UN human rights experts said Mr Duterte's directive for police and the public to kill suspected drug traffickers amounted to "incitement to violence and killing, a crime under international law".
This round of Asean talks comes against the backdrop of tensions over China's territorial ambitions in the South China Sea - the Philippines and the US are key players in that debate.