MANILA - The United States is looking at a "potential disaster" following President Rodrigo Duterte's announcement that he will be shifting the Philippines' alliance to China, a report said.

"From the American viewpoint, Duterte’s flip-flop — assuming it leads to a lasting strategic shift — is a potential disaster," an opinion piece published on the US-based Foreign Policy said Thursday.

"The Philippines has seen a vertigo-inducing change in its foreign-policy orientation since Rodrigo Duterte became president this summer. This crude populist is now transforming the Philippines’ relationship with the United States in a fundamental and worrying manner," it said.

While on a state visit in China, Duterte announced his "separation" from the US, reiterating his disdain for the Western agenda.

With this break, America will find it hard to counter Beijing's moves in the South China and East China seas, according to the article authored by Council on Foreign Relations senior Fellow Max Boot.

"If the Philippines becomes a Chinese satrapy, by contrast, Washington will find itself hard-pressed to hold the 'first island chain' in the Western Pacific that encompasses the Japanese archipelago, the Ryukyus, Taiwan and the Philippine archipelago," it said.

"Defending that line of island barriers has been a linchpin of US strategy since the Cold War. It now could be undone because of the whims of one unhinged leader," it added.

China will then be able to either "neutralize" the Philippines or turn it into a navy base for "menacing US allies" such as Taiwan, Japan and Australia, it said.

"At the very least, the US Navy will find it much harder to protect the most important sea lanes in the world," it said, adding that $5.3 trillion in goods passes through the South China Sea, $1.2 trillion of which is US trade.

What's worse, it said, is the shift doesn't even make sense.

Locked in a territorial dispute with China and being America's oldest and closest ally in Asia, it is unimaginable for the Philippines to shift alliances, it said.

The report cited how US and Filipino troops fought side by side in World War II, how the Philippines hosted two of the largest American military bases overseas and how the US military has been ramping up its presence in the region in response to Chinese aggression.

"Even today China is more foe than friend of America," it added.

There is also no "especially pressing economic case" for the Philippines to realign itself with China since most of its imports comes from the US and its other Asian allies who resent Beijing's aggression, it said.

"This massive geopolitical shift is entirely Duterte’s doing. It cannot be explained any other way. It is a product of his peculiar psychology," said the report, adding he seems to match Chinese President Xi Jinping's leadership style.

"The Duterte-Xi Jinping marriage thus seems like a natural match," it said.

The only good news, it said, was that Duterte's actions could be "undone by a more rational successor, assuming that democracy in the Philippines survives this time of testing."