Beijing said it is ready to take measures to safeguard its national sovereignty and territorial integrity in response to the seriously erroneous acts of collusion between the United States and the Taiwan region.

Authorities in Beijing expressed strong opposition on Thursday to Taiwan leader Tsai Ing-wen's "transit" trip through the US, the latest instance of US meddling in the Taiwan question, which Beijing regards as one of its core interests.

Beijing urged the US to immediately cease all forms of official interaction with Taiwan, stop upgrading its substantive exchanges with the island and stop creating tensions across the Taiwan Straits.

Disrespecting repeated warnings lodged by China in recent days, the US has obstinately allowed Tsai's trip to the country and her high-profile meeting with US House Speaker Kevin McCarthy, who is second in the line of succession to the US presidency.

McCarthy met with Tsai at a bipartisan event at the Ronald Reagan Presidential Library in California on Wednesday, a week after Tsai was given a leadership award in New York. Tsai hailed "unwavering support" from the US, while McCarthy said arms sales to Taiwan should continue.

Wednesday's meeting also included members of a new House select committee that focuses on China, which was launched after McCarthy became speaker.

China's Ministry of Foreign Affairs said that, in essence, the US colluded with Taiwan and connived with attempts by separatists seeking "Taiwan independence" to carry out political activities on US soil, engage in official interaction between the US and Taiwan, and elevate their substantive relations.

"Over a long period of time, the US has obstinately stuck to the strategy of using Taiwan to contain China, and breached the commitments it had made," the ministry said.

The US has relentlessly challenged the bottom line and made provocations in matters such as official interaction with, arms sales to and military collusion with Taiwan as well as helping Taiwan expand its so-called "international space", it said.

The Foreign Affairs Committee of the National People's Congress, China's top legislature, said the meeting has gravely violated the one-China principle and the provisions of the three China-US joint communiques and seriously breached international law and the basic norms governing international relations.

Bao Chengke, assistant director at the Institute for East Asian Studies in Shanghai, said the US is being two-faced by claiming it respects China's core interests, but at the same time, engaging in promoting "one China, one Taiwan".

It was a political act that seriously interfered in China's internal affairs as the speaker, who is a figure with great influence, met with Tsai, which has further aggravated the tension between China and the US and affected peace and stability across the Taiwan Straits, he said.

Although the US has repeatedly stressed that it adheres to the one-China policy and does not support "Taiwan independence", McCarthy referred to Tsai as the "president" of Taiwan, which is another very serious act of provocation, he said.

McCarthy, who spoke at the Ronald Reagan Presidential Library about increasing arms sales to Taiwan, has desecrated the Reagan administration and damaged the image of the late US president, Bao said.

The Reagan administration, which supported peaceful reunification, and the Chinese government reached the August 17 Communique in 1982, in which the US pledged to gradually reduce its arms sales to Taiwan, he said.

The Taiwan Work Office of the Communist Party of China Central Committee said it was another provocative move from Taiwan's Democratic Progressive Party authorities to further collude with the US and cling to its support in order to seek "independence".

Tsai and the DPP authorities, out of selfish motives, have served as a tool for anti-China forces in the US to contain China. However, their actions can never change the fact that Taiwan is part of China, and will only push Taiwan to the brink of war and bring great suffering to the Taiwan compatriots, the office said.

China's Ministry of National Defense said the People's Liberation Army of China will stand by its responsibility and mission and always be on high alert to resolutely safeguard China's sovereignty and territorial integrity, as well as peace and stability across the Straits.

It is the second time Tsai has met with a US lawmaker of such a high rank in the past 12 months. Then US House speaker Nancy Pelosi went to Taiwan in August, a visit that was followed by countermeasures from Beijing, including military drills around the island.

The California stop comes as Tsai returns to the island from a "visit" to Guatemala and Belize. It was part of a 10-day tour to consolidate so-called "ties" with the Central American countries. Honduras announced last month it was establishing diplomatic ties with Beijing, and severed its so-called "diplomatic ties" with Taiwan.

Since Tsai was elected in 2016, several countries have ended "ties" with Taiwan and formed diplomatic relations with Beijing.

Bao, from the Institute for East Asian Studies, said that as Tsai will step down next year, she's embarking on a "graduation tour" to show her "achievements". Her administration has been pushing for arrangements to travel to the US since last year.

Inviting US politicians to Taiwan was an attempt by Tsai to get more photo opportunities with politicians on her US visit, he said.

Tsai's schedule for the US "transit", including a planned speech at the library and meeting with government officials, was curtailed. However, she will not be satisfied and will continue to seek more opportunities for such trips, Bao added.

Wang Ping-chung, a political commentator from Taiwan and chair of the New China Youth Association, said the DPP is attempting to win votes in the upcoming election by colluding with the US to promote cross-Straits confrontation.

He said it was "pathetic" that Tsai, as Taiwan's leader, isn't considering ways to boost the island's economic recovery and is ignoring people's livelihood problems, including shortages of eggs, water and electricity.

"Instead, she only thinks of personal political shows and the DPP's calculations," Wang said.

"What benefits can she bring back to Taiwan by going to the US? It will only make our situation in Taiwan more severe."