Nicaragua broke its longstanding diplomatic ties with Taiwan last week, switching allegiance to Beijing in a recognition of the Chinese Communist Party's One China policy and reducing Taipei's dwindling pool of international allies.

"When democratic countries were holding a democratic summit, China was excluded, China was a target, so China chose this opportunity to set about targeting our diplomatic allies," said Foreign Minister Joseph Wu on the sidelines of a forum on regional security.

Digital Minister Audrey Tang and Taiwan's de facto ambassador in Washington Hsiao Bi-khim represented the island at the Biden administration's "Summit for Democracy" last week. China was not on the US State Department's invited participants list.

"Losing a diplomatic ally is a very painful thing for us," Wu told reporters.

China's foreign ministry said in a statement to Reuters the comments were an attempt to "cover up the failures of separatist activities".

Beijing has increased military and political pressure on Taiwan to accept its sovereignty claims, drawing anger from the democratically ruled island, which has repeatedly said it would not be bullied and has the right to international participation.

Taiwan President Tsai Ing-wen said the island would not bend to pressure or change its determination to uphold democracy and freedom.

"The more successful Taiwan's democracy is, the stronger the international support, and the greater the pressure from the authoritarian camp," she said in Taipei.

China and Nicaragua's move to re-establish diplomatic ties will likely boost Beijing's influence in a part of the world long considered the United States' backyard, angering Washington.

Nicaraguan President Daniel Ortega first cut ties with Taiwan in 1985, but they were re-established with the island in 1990 under then-Nicaraguan President Violeta Barrios de Chamorro.

Wu said all of his colleagues in the foreign ministry "put forth their greatest efforts to maintain these diplomatic ties."

Nicaragua's move to cut ties with Taiwan leaves the island with just 14 formal diplomatic allies, most of them in Latin America and the Caribbean, plus a handful of small state.

Meanwhile, a leading Republican senator on Tuesday demanded that President Joe Biden make amends over a cut in transmission of Taiwan's speech during last week's democracy summit, in what an official called an "honest mistake."

Audrey Tang, a Taiwanese minister, was speaking Friday at Biden's inaugural virtual Summit on Democracy when the broadcast abruptly stopped.

The cut came as Tang appeared before a map that showed Taiwan and China in different colors -- indicative of divergent levels of democracy but possibly interpreted as recognizing the island as independent, a stance that infuriates Beijing.

Senator Marco Rubio, who has championed a hard line on China, accused Biden of trying to appease Beijing and questioned why Taiwan, unlike many participants, was not represented by its head of state.

"All that your administration accomplished in downgrading Taiwan's representation was to once again signal weakness rather than resolve," he said in a letter, quoted by AFP.

He asked Biden to take steps to "make amends" over Taiwan including at a second, in-person democracy summit planned for next year.

A State Department official denied any intent, noting that Tang's presentation can be seen online without interruption.

"There was confusion over the use of screen sharing, which resulted in the video feed being dropped. This was an honest mistake," the official said on condition of anonymity.

"We valued Minister Tang's participation, which showcased Taiwan's world-class expertise on issues of transparent governance, human rights, and countering disinformation."

China denounced the summit, saying the United States was using democracy as a "weapon of mass destruction."

Tensions have risen in recent weeks as China steps up military moves near Taiwan, a self-ruling democracy that Beijing considers a province awaiting reunification.

Biden has sought to emphasize democracy in an implicit shift from his predecessor Donald Trump, who befriended authoritarian leaders, criticized US allies and sought to overturn his election defeat.