The historic alliance between the United States and the Philippines remains "ironclad" despite recent differences, Secretary of State John Kerry said Thursday, alluding to the anti-American rhetoric of populist President Rodrigo Duterte.

"The United States continues to place high value on the close ties that exist between our countries," Kerry said during a swearing-in ceremony for the new US ambassador to Manila, the Korean-born diplomat Sung Kim.

"We continue to recognize our ironclad commitment to the sovereignty, independence and security of the Philippines."

In August, Duterte said in a speech that Kim's predecessor in Manila, Philip Goldberg, was a "son of a bitch" -- a comment that prompted the State Department to demand an official explanation.

Duterte, who accused Goldberg of meddling in Philippine elections, then issued a stream of insults toward the former colonial power, going so far as to refer to President Barack Obama as a "son of a whore."

The fiery Filipino leader later said his comment was "nothing personal" and that he regretted that it had caused such controversy.

But Kerry, who met with Duterte in Manila in late July and said he hopes to return there before the Obama presidency ends in January, struck a diplomatic tone in his remarks Thursday.

"I am confident about the future of our bilateral relationship, notwithstanding the difference here or there about one thing or another," he said. "I am absolutely confident about the ties between our peoples."

Facing the extraordinary insults of the Philippine president, American diplomats have been wary of adding fuel to the fire.

"Democratic elections bring change and we must all have the wisdom to recognize and to adjust to that change," Kerry said.

He added that "the logic of our alliance" -- built around a military partnership forged after World War II and cemented by a mutual defense treaty in 1951 -- remained "as compelling today" as it had ever been.

Until Duterte came to power in June, Manila was one of the closest US allies in Asia, and it was viewed as a vital link in the political "pivot" or "rebalancing" of US policy toward the Asia-Pacific region under Obama.

But late last month Duterte announced his country's "separation" from the United States and called for an end within two years to the presence of "foreign military troops" -- a clear reference to American forces.

Duterte later struck a more conciliatory tone, saying there would be no "severance of ties" with Washington; Manila, he added, would simply pursue a more independent line in foreign affairs while improving its ties to Beijing.

Until the early 1990s, when the Manila government ordered them out, the United States kept thousands of troops at major bases in the Philippines.

But months before Duterte took office, Washington and Manila had announced plans for stepped-up security cooperation, with a "periodic presence" of American troops rotating through the area near the contested South China Sea.