China lashed out on Friday at "irresponsible" US accusations that Beijing was destabilising cross-Strait relations after El Salvador cut ties with Taiwan in favour of the Asian giant.
The White House also scolded the Central American country in a statement released Thursday, warning that the decision was made in a "non-transparent fashion" and affects "the economic health and security of the entire Americas region".
Taiwan and China have been engaged for years in a diplomatic tug-of-war in developing countries, with economic support and other aid often used as bargaining chips for diplomatic recognition.
"The United States will continue to oppose China's destabilisation of the cross-Strait relationship and political interference in the Western Hemisphere," the White House said.
"The El Salvadoran government's receptiveness to China's apparent interference in the domestic politics of a Western Hemisphere country is of grave concern to the United States, and will result in a reevaluation of our relationship with El Salvador."
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 Ing-wen became Taiwan's president in 2016, as her government refuses to acknowledge the island is part of "one China".
China has stepped up its poaching of Taiwan's dwindling official allies, with El Salvador becoming the fifth diplomatic loss under Tsai's presidency and the third this year.
Chinese foreign ministry spokesman Lu Kang said on Friday that El Salvador's decision to adhere to the "one China principle" was "right, proper and above-board".
"Some country takes this and makes irresponsible remarks, is this not interfering in the internal affairs of El Salvador? Isn't this, rather, politically meddling in the region? This is self-evident," Lu told a regular press briefing.
While most countries have established formal ties with Beijing rather than 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.