State Department spokesman John Kirby said the US would seek an explanation, which the US hadn't known were coming.

He described them as "yet another string in some pretty strong rhetoric that we think we believe is at odds with the kind of relationship that we have had and continue to have the Filipino people."

Duterte detailed exactly the kind of "separation" he had in mind Thursday in Beijing, saying, "I announce my separation from the United States, both in military, not maybe social, but economics also."

Damage control?

In an apparent attempt at damage control, Philippine Trade Minister Ramon Lopez said Friday that his country would not stop trade and investment with the US.

"The statement the President made maintains the relationship with the West. What we are saying is that there will be less dependence just on one side of the world," Lopez told CNN.

Daniel Russel, the assistant secretary of state for East Asian and Pacific Affairs, heads to the Philippines Friday as part of a long-planned trip to meet with government officials and do youth outreach.

Key question

Duterte's break from the US on Thursday comes on the heels of other anti-US moves and rhetoric.

He recently called for ending joint military drills with the US, in August he described the US ambassador as "a gay son of a whore," and in September he dismissed President Barack Obama himself as a "son of a bitch" -- in part a response to US criticism of his war on drugs, which killed 2,400 people in two months.

"The key question is whether Duterte is going to rescind our access to our bases in the Philippines," said Robert Manning, a senior fellow at the Atlantic Council.

The Obama administration has built a maritime security initiative with Southeast Asian countries, with a major role for the Philippines, that's aimed in part at helping those countries resist Chinese pressure as Beijing aggressively pursues territorial claims in the South China Sea.

The loss of the Philippines bases would be "a significant blow," said Manning, a former official at the State Department and National Intelligence Council.

"If the Philippines are out -- and they're in the thick of it -- that starts to raise questions about our whole approach" to the region, he concluded.

And if Duterte is realigning his county with China on an economic and even military level, "that would have potentially fundamental implications for the future of the Philippines and Philippine-US relations," according to Jamie Metzl, another Atlantic Council senior fellow.

The US and Philippines operate under a 1951 mutual defense treaty, which Duterte has said he doesn't plan to abrogate. Metzl pointed out that doing so would require approval of the Philippine parliament among "one of the most pro-US and anti-Chinese populations in Asia."

Grandstanding?

Still, there is concern in Washington about what this means. Senior US officials told CNN Duterte may have been grandstanding in China, but it's not yet clear where he's headed.

Duterte's stance may be driven by a resentful sense that the US presence in the Philippines diminishes his country's standing, they said. He may also feel threatened by US questions about his approach to human rights and the drug war.

Sandy Pho, a senior program associate at the Wilson Center, agreed that Duterte resents US power in the Philippines.

"I think Duterte comes into the presidency with this mindset," she said. But she also pointed to a perception in Asia that the US is bogged down in the Middle East.

"The US can't focus" is the sense, she said, "so quite frankly, he's hedging his bets, thinking an overreliance on the United States probably isn't the best bet."

Either way, the Obama administration realizes that the relationship it once had with Manila can no longer be taken for granted.

"He's going to be difficult," one official said.

Publicly, the White House is emphasizing the positive.

Emily Horne, a spokeswoman for the National Security Council, said that the "US-Philippines alliance is built on a 70-year history, rich people-to-people ties, including a vibrant Filipino-American diaspora, and a long list of shared security interests." She also cited strong economic ties, including more than $4.7 billion in US foreign direct investment.

Speaking at the Center for Strategic and International Studies in Washington on October 11, the State Department's Russel said that while there have been "ups and downs" between the two countries, the various links "keep us very closely tethered together."

The US has no problem with the Philippines engaging with China, he added: "We don't regard this as zero sum, but we value our relationship with the Philippines and we want to keep it on even keel."

Obama has focused considerable diplomatic energy on the US relationship with Asia during his two terms, seeing greater engagement with the world's fastest growing economies as a source of American jobs and exports.

The President's "pivot" to Asia was also seen as a way to integrate China into an international order with rules that the US and Western allies have set -- as opposed to rules Beijing might favor -- on core issues like human rights, labor protections and the environment.

All the while, tensions between China and its smaller Asian neighbors over the resource-rich South China Sea have been escalating. The US has urged that these conflicts be solved through international institutions and multilateral talks, and backed Duterte's predecessor when he took China to the Permanent Court of Arbitration in The Hague.

The US held the Philippines as a colony from 1898 until 1946. Around a quarter-century ago, Manila's leaders pushed to have US troops leave the country.

Duterte's predecessor, Benigno Aquino, reversed course on a US security presence in his country in 2014 in the face of China's regional muscle-flexing, signing an agreement that would place more ships in Philippine ports.

Kirby said that the US isn't the only country taken aback by Duterte's move.

"It isn't just the United States who is baffled by this rhetoric," Kirby said. "We have heard from many of our friends and partners in the region who are likewise confused about where this is going, and also, we believe, are trying to learn more on their own about what it portends."