US House of Representatives Speaker Nancy Pelosi arrived in Chinese-claimed Taiwan aboard a US military aircraft late on Tuesday, the first such visit in 25 years and one that risks pushing relations between Washington and Beijing to a new low.

Pelosi and her delegation disembarked from a US Air Force transport plan at Songshan Airport in downtown Taipei and were greeted by Taiwan's foreign minister, Joseph Wu and Sandra Oudkirk, the top US representative in Taiwan.

"Our congressional delegation's visit to Taiwan honors America's unwavering commitment to supporting Taiwan's vibrant democracy," Pelosi said in a statement shortly after landing.

"America's solidarity with the 23 million people of Taiwan is more important today than ever, as the world faces a choice between autocracy and democracy."

Pelosi was on a tour of Asia that includes announced visits to Singapore, Malaysia, South Korea and Japan. Her stop in Taiwan had not been announced but had been widely anticipated.

Chinese warplanes buzzed the line dividing the Taiwan Strait on Tuesday before her arrival, as China's leadership warned against the visit by Pelosi, who is second in the line of succession to the US presidency and a long-time critic of Beijing.

In the latest rhetorical salvo, Chinese Foreign Minister Wang Yi said on Tuesday that US politicians who "play with fire" on the Taiwan issue will "come to no good end".

The United States said on Monday it would not be intimidated by what it called Chinese "sabre-rattling".

Most of Pelosi's planned meetings, including with President Tsai Ing-wen, were scheduled for Wednesday, a person familiar with her itinerary said. Four sources said she was scheduled on Wednesday afternoon to meet a group of activists who are outspoken about China's human rights record.

Pelosi, 82, is a close ally of US President Joe Biden, both being members of the Democratic Party, and has been a key figure in guiding his legislative agenda through the US Congress.

Taiwan's foreign ministry said it had no comment on reports of Pelosi's travel plans, while her office had also kept silent.

On Tuesday night, Taiwan's tallest building, Taipei 101, lit up with messages including: "Welcome to Taiwan", "Speaker Pelosi", "Taiwan (heart) USA".

With tensions already high, several Chinese warplanes flew close to the median line dividing the Taiwan Strait on Tuesday morning before leaving later in the day, a source told Reuters. Several Chinese warships have also sailed near the unofficial dividing line since Monday and remained there, the source said.

The Chinese aircraft repeatedly conducted tactical moves of briefly "touching" the median line and circling back to the other side of the strait while Taiwanese aircraft were on standby nearby, the person said.

Neither side's aircraft normally cross the median line.

Four US warships, including the aircraft carrier USS Ronald Reagan, were positioned in waters east of Taiwan on what the US Navy called routine deployments. The carrier had transited the South China Sea and was now in the Philippines Sea, east of Taiwan and the Philippines and south of Japan, a US Navy official told Reuters.

It was operating with the guided missile cruiser USS Antietam and destroyer USS Higgins, with the amphibious assault ship USS Tripoli also in the area.

US House of Representatives Speaker Nancy Pelosi meets Malaysia's Parliament Speaker Azhar Azizan Harun in Kuala Lumpur.

Since last week, China's People's Liberation Army has conducted various exercises, including live-fire drills, in the South China Sea, Yellow Sea and Bohai Sea, in a show of Chinese military might.

China views visits by US officials to Taiwan as sending an encouraging signal to the pro-independence camp on the democratic, self-governed island. Beijing considers Taiwan to be part of its territory and has never renounced using force to bring the island under its control. Taiwan rejects China's sovereignty claims and says only its people can decide the island's future.

The United States has no official diplomatic relations with Taiwan but is bound by American law to provide the island with the means to defend itself.

'State Provocateur'

Russia — itself locked in confrontation with the West over its invasion of Ukraine — also chimed in on Pelosi's expected visit. Maria Zakharova, Russia's foreign ministry spokeswoman, said the visit was a provocative US attempt to pile pressure on China, a country with which Russia has forged a strong partnership in recent years.

"The USA is a state provocateur," Zakharova said. "Russia confirms the principle of 'one China' and opposes the independence of the island in any form."

Earlier on Tuesday, Pelosi visited Malaysia, having begun her Asia tour in Singapore on Monday. Her office said she would also go to South Korea and Japan but made no mention of a Taiwan visit.

Taiwan's Defence Ministry said it had a full grasp of military activities near Taiwan and that it would dispatch forces appropriately in reaction to "enemy threats".

China's defence and foreign ministries did not respond to requests for comment.

In the southeastern Chinese city of Xiamen, which lies opposite Taiwan and has a large military presence, residents reported sightings of armoured vehicles.

Chinese social media was abuzz with both trepidation about potential conflict and patriotic fervour.

"Faced with reckless US disregard of China's repeated and serious representations, any countermeasures taken by the Chinese side will be justified and necessary, which is also the right of any independent and sovereign country," foreign ministry spokesperson Hua Chunying told a daily briefing in Beijing.

During a phone call last Thursday, Chinese President Xi Jinping warned Biden that Washington should abide by the one-China principle and "those who play with fire will perish by it". Biden told Xi that US policy on Taiwan had not changed and that Washington strongly opposes unilateral efforts to change the status quo or undermine peace and stability across the Taiwan Strait.

White House national security spokesperson John Kirby said on Monday that Beijing's responses could include firing missiles near Taiwan, large-scale air or naval activities, or further "spurious legal claims" such as China's assertion that the Taiwan Strait is not an international waterway.

"We will not take the bait or engage in sabre-rattling. At the same time, we will not be intimidated," Kirby said.

Bonnie Glaser, a Taiwan expert at the German Marshall Fund of the United States, told reporters in a call that the damage to American-Chinese relations done by the Pelosi visit would be hard to repair.

"We all know how bad this relationship has been in the past year. And I just think that this visit by Nancy Pelosi is just going to take it to a new low," Glaser said. "And I think that it's going to be very difficult to recover from that."