The US government on Monday summoned the charge d'affaires of the Philippine Embassy to clarify what it called "inappropriate comments" made by Philippine President Rodrigo Duterte about US Ambassador Philip Goldberg.

"We've seen those inappropriate comments made about Ambassador Goldberg," said US State Department spokeswoman Elizabeth Trudeau.

But she declined to disclose the details of the conversation between US officials and Patrick Chuasoto, the Philippines charge d'affaires in Washington.

"I'm not going to read out that detailed conversation, but it was specifically on those remarks," Trudeau said.

Duterte reportedly called Goldberg a "gay ambassador" and used expletives to describe the ambassador.

"I am pissed with him," Duterte said. "He meddled during the election, giving statements here and there."

Goldberg told the US media in April that the United States did not condone anyone who either degrades women or trivializes issues as serious as rape, referring to a joke made by Duterte during his campaign about the rape and murder case involving Australian missionary Jacqueline Hamill in 1989.

The United States on Monday also expressed concern over the extrajudicial killings of drug suspects. Duterte was swept to power in May after pledging to end crime using the same "shoot-to-kill" methods he employed as mayor of the southern city of Davao.