US President Barack Obama and Philippine President Rodrigo Duterte met informally on Wednesday in a holding room before attending a gala dinner at a regional summit in Laos, White House and Philippine officials said.

Philippine President Rodrigo Duterte arrives at the ASEAN Summit family photo while US President Barack Obama chats with the Sultan of Brunei Hassanal Bolkiah in Vientiane, Laos Sept 7. [Photo/Agencies]

The meeting came after Duterte on Monday called Obama a "son of a b----" and, while speaking to reporters, warned Obama not to question killings that have occurred during his crackdown on suspected drug dealers and users. More than 2,000 people have been killed in the crackdown since he took office on June 30.

That caused Obama to cancel a formal meeting scheduled for Tuesday. Obama, in turn, called Duterte "a colorful guy". On Tuesday, Duterte expressed regret over the remarks.

A Philippine Department of Foreign Affairs spokesman, Charles Joe, said it was a mutually agreed meeting, but that he had no details of what was discussed.

Philippine Foreign Secretary Pefecto Yasay told The Associated Press that the leaders had met:

"They met at the holding room and they were the last persons to leave the holding room. I can't say how long they met. It all springs from the fact the relationship between the Philippines and the United States is firm, very strong. The basis for this relationship is historical and both leaders realize this. And I'm very happy that it happened"

Obama and Duterte entered the dinner venue separately, and were seated far apart and did not interact with each other during the dinner that lasted an hour and 20 minutes.