The de facto US ambassador to Taiwan said Thursday Washington would reassess its relations with El Salvador after the Central American country switched ties from Taipei to Beijing this week.
The US is "deeply disappointed" at the break in ties between Taiwan and El Salvador, said Brent Christensen, the new director of the American Institute in Taiwan, in a meeting with Taiwan's President Tsai Ing-wen.
Taiwan and China have been engaged for years in a diplomatic tug-of-war in developing countries. Economic support and other aid are often used as bargaining chips for diplomatic recognition.
"El Salvador's receptiveness to China's apparent interference in domestic politics in western hemisphere countries is of grave concern to the United States," Christensen said at the meeting in Taipei.
The move "will result in a reassessment of our relationship with El Salvador", he added.
Christensen also reaffirmed US support for Taiwan to "resist efforts to constrain its appropriate participation on the world stage".
Beijing sees self-ruling, democratic Taiwan as part of its territory to be brought back into the fold and has not ruled out using force to do so.
Relations between Taipei and Beijing have worsened since Tsai came to power in 2016 as her government refuses to acknowledge that Taiwan is part of "one China".
As a result, China has stepped up its poaching of Taiwan's dwindling official allies.
El Salvador became the fifth diplomatic loss under Tsai's presidency and the third this year.
But while most countries have established formal relations with Beijing rather than rival Taipei in recent decades, Taiwan still has unofficial relationships with powerful countries.
Its most important unofficial ally is the US, which remains Taiwan's leading arms supplier, despite switching diplomatic recognition to Beijing in 1979.
Ties have warmed between the US and Taiwan in recent months, incensing Beijing.
Some analysts saw the poaching of El Salvador as an indirect slap in the face to the Trump administration and a warning against further friendly overtures.
The cutting of ties came just after Tsai had wrapped up a Latin American tour with high-profile stopovers in the US.
In her meeting with Christensen, Tsai said China was suppressing Taiwan "with every conceivable means".
She called on "like-minded countries to jointly express concern" over China's actions.
Taiwan is often shut out of international forums under Beijing pressure and some global firms have caved to requests to list Taiwan as part of China on their websites.