By Yimou Lee and Martin Quin Pollard
TAIPEI/BEIJING (Reuters) - U.S. House of Representatives Speaker Nancy Pelosi was set to visit Taiwan on Tuesday, three sources said, as the United States said it wouldn't be intimidated by Chinese threats to never "sit idly by" if she made the trip to the self-ruled island claimed by Beijing.
Pelosi, who began an Asia trip earlier on Monday in Singapore, was due to spend Tuesday night in Taiwan, three people briefed on the matter said.
Taiwan's foreign ministry said it had no comment on reports of Pelosi's travel plans, but the White House - which wouldn't confirm the trip - said she had the right to go.
China's responses could include firing missiles near Taiwan, large-scale air or naval activities, or further "spurious legal claims" such as Beijing's assertion that the Taiwan Strait is not an international waterway, White House national security spokesman John Kirby told reporters in Washington.
"We will not take the bait or engage in sabre rattling. At the same time, we will not be intimidated," Kirby said.
Amid widespread speculation over whether she would make a stop in Taiwan, Pelosi's office said on Sunday that she was leading a congressional delegation to the region that would include visits to Singapore, Malaysia, South Korea and Japan. It did not mention Taiwan.
One source told Reuters that the United States had informed some allies about Pelosi's visit to Taiwan. Two other sources said Pelosi was scheduled to meet a small group of activists who are outspoken about China's human rights record during her stay in Taiwan, possibly on Wednesday.
Chinese foreign ministry spokesman Zhao Lijian said on Monday that it would be "a gross interference in China's internal affairs" if Pelosi visits Taiwan, and warned that it would lead to "very serious developments and consequences."
"We would like to tell the United States once again that China is standing by, the Chinese People's Liberation Army will never sit idly by, and China will take resolute responses and strong countermeasures to defend its sovereignty and territorial integrity," Zhao told a regular daily briefing.
Asked what kind of measures the PLA might take, Zhao said: "if she dares to go, then let us wait and see."
China views visits by U.S. officials to Taiwan 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. Republican Newt Gingrich was the last House speaker to visit Taiwan, in 1997.
A video by the People's Liberation Army's Eastern Theater Command, which showed scenes of military exercises and preparations and was posted on state media sites on Monday evening, urged troops to "stand by in battle formation, be ready to fight upon command, bury all incoming enemies."
The White House has dismissed China's rhetoric as groundless and inappropriate.
Kirby said that nothing about Pelosi's possible trip changed U.S. policy toward Taiwan, and that Beijing was well aware the division in 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 the White House briefing.
During a phone call last Thursday, Chinese President Xi Jinping warned U.S. 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.
Spokesman for China's embassy in Washington Liu Pengyu struck a measured tone with reporters on Friday, a day after Biden spoke with Xi, saying one of the goals of dialogue was to ease tension over Taiwan.
"I think both China and the U.S. don't want a conflict there in the Taiwan Strait," Liu said.
On Monday, Taiwan Premier Su Tseng-chang did not directly respond when asked whether Pelosi will visit but told reporters visits by "distinguished foreign guests" were welcome.
Shi Yinhong, an international relations professor at Renmin University in Beijing, said that if Pelosi visits Taiwan it would prompt the strongest counter-measures by Beijing in years, but he did not expect that to trigger major military conflict.
"China has reiterated in no ambiguous terms its opposition to Taiwan separatism. The U.S. has reiterated many times its one-China policy has not changed and that it is against any change to the status quo by either side of the Taiwan Strait," he said.
"Unless by accident, I am sure neither side would intentionally take military action that could lead to a major security risk."
On Monday, Pelosi and her delegation met with Singapore Prime Minister Lee Hsien Loong, discussing issues including cross-strait relations, the Ukraine war and climate change, Singapore's foreign ministry said.
"PM Lee highlighted the importance of stable US-China relations for regional peace and security," it said.
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.
Last Wednesday, Biden told reporters he thought the U.S. military believed a Pelosi visit to Taiwan was "not a good idea right now."
(Reporting by Chen Lin in Singapore, Yimou Lee in Taipei, Martin Quin Pollard and Yew Lun Tian in Beijing, and Jeff Mason, Michael Martina and Doina Chiacu in Washington; Writing by Tony Munroe; Editing by Simon Cameron-Moore, Chizu Nomiyama and Grant McCool)