China has condemned the US for scrapping curbs on interactions with Taiwan officials, saying nobody could prevent the country's "re-unification", while Taiwan's foreign minister hailed the US move as a sign of "global partnership".
US Secretary of State Mike Pompeo announced the change on Saturday, in the waning days of the Republican Trump administration before Democrat Joe Biden assumes the presidency on January 20.
China, which claims democratic Taiwan as its own territory, said it was "resolutely opposed" to the decision and strongly condemned it.
"The Chinese people's resolve to defend our sovereignty and territorial integrity is unshakable and we will not permit any person or force to stop the process of China's re-unification," foreign ministry spokesman Zhao Lijian told reporters on Monday.
"Any actions which harm China's core interests will be met with a firm counterattack and will not succeed," he added.
Taiwan welcomed Pompeo's decision, with Foreign Minister Joseph Wu telling reporters it was a "big thing for the elevation of Taiwan-US relations".
He added, "Taiwan-US relations have been elevated to a global partnership. The foreign ministry will not let our guard down and hopes to continue to boost the development of Taiwan-US ties."
Like most countries, the US has no official ties with Taiwan, though it is bound by law to provide the island with the means to defend itself. Under President Donald Trump it has ramped up arms sales and sent senior officials to Taipei.
But Pompeo's decision means, for example, that Taiwan officials will be able to hold meetings at the State Department or White House rather than in non-official locations elsewhere, such as hotels.
Chinese state media lambasted the decision.
The widely read Global Times tabloid, published by the ruling Communist Party's official People's Daily, said China must send a "stern warning" to Taiwan.
"Those on the island of Taiwan must not take for granted that they can seek secession with the help of the last-ditch madness of an administration abandoned by the Americans," the paper added.
On Wednesday, the US ambassador to the United Nations, Kelly Craft, will arrive in Taipei for a three-day visit that China has denounced.
Wu said both he and President Tsai Ing-wen would meet Craft on Thursday, adding that discussing ways to promote Taiwan's international participation would be a topic during the visit.
Craft's visit is highly symbolic, as Taiwan is not a member of the United Nations, or most global bodies, because of China's objections. Beijing says only it has the right to speak for Taiwan on the international stage.
Late on Sunday, Johnny Chiang, the chairman of the main opposition Kuomintang party, which traditionally favours close China ties, said a key test would be whether Biden's government maintained Pompeo's changes.
"Let's not become a bargaining chip in the game between the United States and mainland China," he said.