TAIPEI: U.S. House of Representatives Speaker Nancy Pelosi was expected to arrive in Taipei later on Tuesday, people briefed on the matter said, as several Chinese warplanes flew close to the median line dividing the Taiwan Strait, a source told Reuters.

China has repeatedly warned against Pelosi going to Taiwan, which it claims as its own, while the United States said on Monday that it would not be intimidated by Chinese "sabre rattling."

In addition to Chinese planes flying close to the median line of the strait, several Chinese warships have sailed near the unofficial dividing line since Monday, the source told Reuters. Chinese warships and aircraft "squeezed" the median line on Tuesday morning, the source said, an unusual move the person described as "very provocative."

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

The Chinese planes left the area in the afternoon, but the vessels remained, the person said.

Neither side's aircraft normally cross the median line.

Taiwan's Defence Ministry said in a statement it has a full grasp of the military activities and will appropriately dispatch forces in reaction to any "enemy threats".

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

Meanwhile, the website of Taiwan's presidential office was targeted by an overseas cyber attack on Tuesday and was at one point malfunctioning, a source briefed on the matter said. The office later confirmed the attacks.

In the southeastern Chinese city of Xiamen, which lies opposite Taiwan and is home to a large military presence, residents reported sightings of armoured vehicles on the move on Tuesday and posted pictures online. The images could not immediately be verified by Reuters.

Chinese social media was abuzz with both trepidation about potential conflict and patriotic fervour, and Pelosi's trip was the top-trending item on the Twitter-like Weibo.

WEDNESDAY MEETINGS

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 Pelosi was scheduled to meet a group of activists who are outspoken about China's human rights record on Wednesday afternoon.

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

Taiwan's foreign ministry said it had no comment on reports of Pelosi's travel plans.

China reiterated its opposition to a Taiwan visit by Pelosi.

"Faced with reckless U.S. 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," spokesperson Hua Chunying told a daily briefing in Beijing.

'SPURIOUS' CLAIMS

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, White House national security spokesperson John Kirby said on Monday.

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

China views visits by U.S. officials to Taiwan, a self-ruled island claimed by Beijing, as sending an encouraging signal to the pro-independence camp in the island. Washington does not have official diplomatic ties with Taiwan but is bound by U.S. law to provide the island with the means to defend itself.

A visit by Pelosi, who is second in the line of succession to the U.S. presidency and a long-time critic of China, would come amid worsening ties between Washington and Beijing.

'CHESS PIECE'

Kirby said that nothing about Pelosi's possible trip changed U.S. policy toward Taiwan, and that Beijing was well aware the division of powers within the U.S. government meant Pelosi would make her own decisions about the visit.

"The speaker has the right to visit Taiwan," he told a White House briefing on Monday.

During a phone call on July 28, Chinese President Xi Jinping warned President Joe Biden that Washington should abide by the one-China principle and "those who play with fire will perish by it".

Biden told Xi that U.S. 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.

Beijing has never renounced using force to bring Taiwan under its control. Taiwan rejects China's sovereignty claims and says only its people can decide the island's future.

Dozens of protesters from pro-China groups gathered on Tuesday outside Taipei's Grand Hyatt hotel, where local media speculated Pelosi might stay. "Get out! Trouble maker Nancy Pelosi!” one sign carried by a protestor read, as around 50 police officers looked on.

Taiwan residents, accustomed to being caught in the middle of China-U.S. tensions, expressed mixed views on a Pelosi visit.

Yang Hsing-ruel, a 22-year-old university student, said he was not overly worried about China's tough rhetoric and hopes any visit bolsters ties between Taiwan and the United States.

Fellow student Chang Yun-fan, 22, had few expectations. "In the end we are just a chess piece in someone else’s game," he said.