Pelosi lands in Malaysia as China rages over Taiwan
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Beijing views Taiwan as its territory and has indicated through repeated warnings that it would view the visit as a major provocation.
Pelosi landed at a Malaysian air force base ahead of meetings with the prime minister and the speaker of the lower house of parliament, state news agency Bernama reported.
While President Joe Biden's administration is understood to be opposed to a Taiwan stop by Pelosi, White House National Security Council spokesman John Kirby said she was entitled to go where she pleased.
"The speaker has the right to visit Taiwan," he told reporters.
"There is no reason for Beijing to turn a potential visit consistent with longstanding US policies into some sort of crisis."
He said Pelosi was travelling on a military aircraft and that while Washington does not fear a direct attack, it "raises the stakes of a miscalculation".
Premier Su Tseng-chang did not confirm the visit on Tuesday when asked by reporters but thanked Pelosi for her support.
And Taiwanese newspaper Liberty Times cited unnamed sources as saying Pelosi would land on the island Tuesday night, then meet Tsai the next day before departing in the afternoon.
More warnings from China
Taiwan's 23 million people have long lived with the possibility of an invasion, but the threat has intensified under Chinese President Xi Jinping.
In a call with Biden last week, Xi warned the United States against "playing with fire" on Taiwan.
And on Monday, China's ambassador to the United Nations, Zhang Hun, said such a visit would be "very much dangerous, very much provocative".
If it happens, "China will take firm and strong measures to safeguard our sovereignty and territorial integrity", he said.
American officials often make discreet visits to the island to show support, but a Pelosi trip would be higher-profile than any in recent history.