Taipei, Taiwan - Taiwan's president-elect Lai Ching-te hailed the island's "solid partnership" with Washington as he welcomed a US delegation Monday, which China said it "firmly opposed."

While Taiwan is not diplomatically recognized by the United States, Washington is a partner and its top weapons provider.

The island lost one of its few formal diplomatic allies on the same day, as Pacific nation Nauru unexpectedly announced it was severing ties and switching allegiance to Beijing.

The switch, just days after Taiwan's presidential election, means only 12 nations now formally recognize Taiwan, which Beijing claims as part of China.

Nauru's announcement overshadowed the visit by the unofficial delegation sent by US President Joe Biden's administration to congratulate Lai.

The Nauru government said it would no longer recognize Taiwan "as a separate country" but "rather as an inalienable part of China's territory" – echoing Beijing's position on the island.

Taiwan cut ties in return to "safeguard our national dignity," and accused Beijing of buying Nauru off.

"China actively reached out to Nauru politicians and used economic aids to induce the country to switch diplomatic recognition," said deputy foreign minister Tien Chung-kwang.

Taiwan's Presidential Office called it a "wrong decision" and accused China of wielding "diplomatic repression (as) a retaliation against democratic values."

But China's foreign ministry said Beijing's resumption of ties with Nauru "reflects the sentiments of the people."

At Taipei's Diplomatic Headquarters – a building that houses most of Taiwan's foreign embassies – Nauru's flag was removed.

Losing Nauru comes as an early blow to Lai just two days after voters defied Beijing's repeated calls not to elect him.

In the run-up to the poll, Chinese officials slammed Lai as a dangerous separatist who would take Taiwan down the "evil path" of independence.

Beijing, which claims the self-ruled island as its territory and has never renounced force to bring it under its control, insisted that the vote did not change the fact the island was part of China.

Lai said Monday freedom and democracy "are the most valuable assets for the Taiwanese people" during his meeting with the delegates at his party's headquarters.

"They are also the core values Taiwan and the United States share and the foundation for the long-term stability in Taiwan-US partnership," he said, adding that strong US support "is of great significance to Taiwan."

Before meeting Lai, the delegation met with President Tsai Ing-wen, who said their visit highlighted the "close and staunch" US-Taiwan partnership.

China said it was "firmly opposed" to all official exchanges between the United States and Taiwan.

China vehemently objects to anything that even suggests official recognition of Taiwan.

The delegation consists of a former US national security adviser and a former deputy secretary of state, and was led by the chair of the American Institute of Taiwan – the de facto US embassy for the island.

Lai, of the ruling Democratic Progressive Party (DPP), had vowed to defend the island from China's "intimidation," and Taipei's foreign ministry told Beijing to accept the result.

He reiterated to the delegates that under his future administration, "Taiwan will continue to defend peace and stability across the Taiwan Strait under the foundation built by President Tsai Ing-wen."

The last time a US delegation visited immediately after an election was in 2016, after Tsai's win, to meet her incoming team and the losing candidates.

Since then, China cut off all high-level communications, as Tsai and her party have defended Taiwan's sovereignty by saying the island is "already independent."

Beijing maintains a military presence around Taiwan, sending in warplanes and naval vessels near-daily, which conflict experts call "gray zone" actions that stop short of an outright act of war.

But the saber-rattling has upped worries of possible accidents escalating into full-blown conflict.

Under Tsai's two-term administration, Taiwan has greatly bolstered its defense resources, buying fighter jets and building its own submarine.

Her deputy Lai has vowed to follow the same policy path.

But he has been more outspoken in the past on the issue of independence, though he has moderated his comments to fit the party line in the lead-up to the election.

His win in Saturday's vote delivered an unprecedented third term for the DPP, but they no longer have their majority in the legislature, losing 12 seats, while the main opposition Kuomintang party gained 14 seats.