The two leaders sat down together at the Ronald Reagan Presidential Library in Simi Valley, approximately 55 kilometres (34 miles) from Los Angeles.

The closed-door meeting makes McCarthy the highest-ranking official to meet with Taiwan’s president on US soil since 1979, the year Washington established diplomatic relations with China.

McCarthy called Tsai “a great friend to America” and added: “I am optimistic we will continue to find ways for the people of America and Taiwan to work together to promote economic freedom, democracy, peace and stability.”

Tsai thanked McCarthy for his hospitality, calling it warm, like the California sunshine, and also thanked the rest of the congressional delegation, saying: “I am so pleased.”

In a press conference later in the afternoon, McCarthy reaffirmed his support for continuing dialogue with Taiwan.

“The friendship between the people of Taiwan and America is a matter of profound importance to the free world, and it is critical to maintain economic freedom, peace and regional stability,” he said.

Tsai spoke after McCarthy, thanking him and other US congressional leaders for meeting with her.

“Their presence and unwavering support reassure the people of Taiwan that we are not isolated and we are not alone,” Tsai said, later adding: “We are stronger when we are together.”

Acknowledging the meeting’s setting in the Reagan Presidential Library, she quoted the former Republican president as saying that freedom needed to be actively defended.

“It is no secret that today the peace that we have maintained and the democracy that we have worked hard to build are facing unprecedented challenges,” Tsai said. “We once again find ourselves in a world where democracy is under threat. And the urgency of keeping the beacon of freedom shining cannot be understated.”

A bipartisan group of politicians were present at the meeting, according to a statement from McCarthy’s office in the lead-up to the visit. A small crowd also gathered outside the library, demonstrating on behalf of pro-Taiwan and pro-China views.

US Secretary of State Antony Blinken has called Tsai’s transit through the US “normal”, noting that she has made six similar stopovers in recent years.

“Beijing should not use the transit as an excuse to take any actions to ratchet up tensions, to further push it changing the status quo,” Blinken said Wednesday during a news conference in Brussels.

However, a spokesperson for China’s Taiwan Affairs Office, Zhu Fenglian, has warned that Beijing will “resolutely fight back” in the wake of any meeting between McCarthy and Tsai.

In a press conference on Wednesday, White House spokesperson Karine Jean-Pierre emphasised that Tsai’s transit is part of “a long tradition” of Taiwan’s leaders passing through the US.

But she reiterated Blinken’s message that the stopover should not be a “pretext to overreact”.

“Our channels of communications are open,” Jean-Pierre said. “And we have had a consistent message that has urged restraint. And in recent days, we have conveyed directly to the Chinese at high levels that escalation is uncalled for. And so we’ll continue to keep those channels of conversations open.”

The meeting comes at the tail end of a trip Tsai made to Central America, to shore up support in Belize and Guatemala, two of Taiwan’s 13 remaining diplomatic allies.

But her travels took her through New York late last month. Her “transit” through California on Wednesday is part of her return journey to Taiwan.

The unofficial US stops, however, have triggered a strong response from China, which considers Taiwan an inalienable part of its territory and therefore opposes any state-to-state relations with the island.

On Monday, China’s consulate in Los Angeles reiterated that the meeting with McCarthy would destabilise “the political foundations of China-US relations”, as well hurting “the national feelings of 1.4 billion Chinese people”.

The US, however, has maintained an unofficial policy of “strategic ambiguity” toward Taiwan and questions of its sovereignty. It does not maintain official diplomatic relations with the island.

But while it “acknowledges” Beijing’s “One China” policy, Washington rejects any use of force to settle the dispute over Taiwan’s independence. It also offers the island support through military resources and trade.

For his part, McCarthy, a Republican, has been outspoken in his opposition to China’s leadership. In December, he collaborated on a Fox News editorial that stated “the greatest threat to the United States is the Chinese Communist Party”.

He has been a strong advocate for “aiding Taiwan in its self-defence” against China’s “aggression”, even proposing a visit to Taiwan after being elected speaker of the House of Representatives.

That position makes him the third-highest-ranking politician in the US government.

China, however, has repeatedly warned against any meetings between US and Taiwan leadership.

When former House Speaker Nancy Pelosi made a trip to the island last August, meeting with Tsai, China responded with war games around the island, including missile launches in the Taiwan Strait.

The White House has sought to downplay the significance of Tsai’s stops in the US. Her transit in New York last week included a speech at the Hudson Institute think-tank, as well as meetings with Democratic House minority leader Hakeem Jeffries and members of the Senate Armed Services Committee, including Jodi Ernst, Dan Sullivan and Mark Kelly.