President Rodrigo Duterte has apologized to former United States President Barack Obama for telling him to "go to hell" and calling him "son of a bitch" when he was still in power.

In a speech with the Filipino community in Israel on Sunday (Israel time), Duterte said he only berated Obama openly because when the latter was still president, he was "not a civilized person" for openly hitting the Philippine war on drugs.

If Obama had complaints against him, he should have gone to the United Nations, Duterte said.

But Duterte added, "well then it would be appropriate also to say at this time to Mr. Obama that you are now a civilian and I am sorry for uttering those words."

"No, it was just a plain talkatise also like yours. We have learned our lessons very well," he said.

"So if it is to your heart to forgive, you forgive. I have forgiven you," the President added.

Duterte, meanwhile, reiterated that his relationship with US President Donald Trump, Obama's successor, is very warm.

"Trump is a good friend of mine," he said, citing the US president even congratulated him and expressed support on his administration's campaign against illegal drugs.

Duterte's war on drugs has been highly criticized locally and abroad due to the increasing number of alleged extrajudicial killings.

Government data showed that over 4,000 individuals allegedly involved in illegal drugs have been killed in the police operations since July 1, 2016 under the Duterte administration. Celerina Monte/DMS