(CNN) Rodrigo Duterte, the Philippines' firebrand president, is not known for being diplomatic.

He recently said he doesn't care about human rights and ordered the military to "shoot to kill" drug dealers. He even cursed Pope Francis for causing traffic during his visit to Manila last year.

And now Duterte has used a gay slur against the US ambassador to his country.

Duterte was heavily criticized during his presidential campaign earlier this year for a statement he made indicating he had wanted to rape a missionary.

After US envoy Phil Goldberg objected to Duterte's remark, Duterte accused the ambassador of meddling in the election and on Friday called him a "gay son of a bitch."

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The State Department summoned Manila's envoy to Washington to clarify the remarks. But officials have been tight-lipped about the conversation, as the diplomatic spat comes at a sensitive time in relations with the Philippines, a key ally and former US colony.

Under Duterte's predecessor, Benigno Aquino III, Washington and Manila sought to deepen their long-standing military alliance as a counterweight to China, as well as cooperation on combating terrorism.

The two countries strengthened a defense pact to increase the US military presence in the Philippines and have held joint military exercises. Most recently, the US voiced its support for an international tribunal's ruling against China over disputes in the South China Sea.

Washington fears Manila's foreign policy alignment, traditionally one of America's closest in Asia, could shift under Dutarte, who swept to power in a landslide in May.

The new president has signaled he would be willing to have bilateral talks with China over the South China Sea and favors greater economic cooperation with Beijing, which could have implications for its relationship with Washington.

The US has also voiced concern, shared by human rights groups, about Duterte's bloody crackdown on alleged drug dealers. More than 4,400 suspected drug dealers have been arrested since Duterte took office and more than 400 killed.

On Monday, State Department spokeswoman Elizabeth Trudeau said the Obama administration was "concerned by these detentions, as well as the extrajudicial killing of individuals suspected to be involved in drug activity in the Philippines."

This week Duterte publicly named more than 150 officials and accused them of being linked to drugs in the Philippines. In reading the list he said the officials would have their day in court but necessarily equal protection from his rhetoric, saying, "my mouth has no due process."

Hours earlier Duterte vowed to keep his "shoot-to-kill" order against drug dealers "until the last day of my term, if I'm still alive by then."

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He also threatened to declare martial law if the country's judiciary interferes with his war on drugs.

"I don't care about human rights, believe me," Dutarte said, according to official transcripts released by his office.

Despite Washington's trepidation over Dutarte, Gregory Poling, a fellow following South Asia at the Center for International Strategic Studies, doesn't expect his offensive comments to alter the course of the decades-long alliance, even if it was stronger under Aquino.

"Both of our national interests are pretty closely linked, which is what drove this kind of a revitalization of the alliance under Aquino," he said. "Personalities helped, but underlying that are real national interests that are emerging."

But US diplomats managing the relationship with the Philippines now "have a whole new task on their hands" with Duterte in office, he said, warning, Washington must tread carefully to avoid sending him into Beijing's arms.

"Duterte is a loose cannon," Poling said. "He's going to say things that are going to rub people the wrong way, and we have to be very careful about the things we say because obviously he gets rubbed the wrong way rather easily."