"Some countries (that switched diplomatic allegiance to Beijing) are coming back to us and saying they didn't get what China promised, they didn't get what China promised financially and they're not getting what China promised politically", Taiwan Foreign Minister Wu told CNN in an interview last month.
"El Salvador has made the choice to commit itself to one China with no pre-conditions, thus standing with most countries in the world", Wang said in comments that seemed meant to pre-empt Taiwanese accusations that China had bought the small Latin American nation's loyalty.
Democratic Taiwan is one of China's most sensitive territorial issues and Tsai's USA stopovers in the United States, a customary practice for Taiwanese presidents, came amid an increasingly bitter trade war between Beijing and Washington. Both Burkina Faso and the Dominican Republic cut ties with Taiwan in May.
Relations between Taipei and Beijing have worsened since Tsai came to power as her government refuses to acknowledge that Taiwan is part of "one China".
"As a responsible member of the global community Taiwan will not engage in dollar nor debt-trap diplomacy with China", Wu said.More news: Milner and Mane strike to beat Palace
Chinese Foreign Minister Wang Yi met with his Salvadoran counterpart in Beijing on Tuesday morning and signed a joint communique to establish formal diplomatic relations.
Since the beginning of this year, rumors have leaked out suggesting the Holy See might be close to a decision of establishing diplomatic relations with Beijing in favor of Taipei, but Wu said Taiwan still hoped the Vatican could remain its ally for the long term.
Taiwan's formal name is the Republic of China, whose government fled to Taiwan in 1949 after losing a civil war with the Communists.
In Beijing, Castaneda said it was a strategic decision his government made to "create conditions to change the historical standing of our country and to really elevate the livelihood of our people".
Taiwan has accused China of luring its friends away with offers of generous aid packages.
The news comes as Tsai wrapped up a high-profile trip to Latin America, including stops in the United States, that drew anger from China.More news: Australia boosts aid by AUS$1.8 billion for drought-hit farmers
Relations between China and Taiwan have reached a low under Tsai, who belongs to the Democratic Progressive party, which advocates independence for the island.
El Salvador is the third Taiwanese ally this year to switch recognition to China, reportedly under promises of large infrastructure projects, and the fifth since Tsai became president in 2016.
The Chinese government has confirmed that it has established formal relations with El Salvador. "We will be stronger and more united, and this will strengthen Taiwan's determination to go out". "We continue supporting the Salvadoran people", she wrote without elaborating.
Only 17 nations now recognise Taiwan diplomatically.
Some analysts say Chinese President Xi Jinping, one of the most powerful Chinese leaders in decades, seems determined to bring Taiwan under Beijing's control during his time in office, which would place him in the history books alongside Mao Zedong.More news: Apple planning fall release for new Mac mini aimed at pros