Taiwan is "preparing for war" as officers are called back from leave and forces are mobilised.
Taiwanese National Defense Ministry has reportedly cancelled some officers' and soldiers' leave and briefed air defence forces to "prepare for war".
It comes as US House of Representatives Speaker Nancy Pelosi was set to kick-off a tour of four Asian countries on Monday in Singapore amid intense speculation that she may risk the wrath of Beijing by also visiting Taiwan, the self-ruled island claimed by China.
China warned that its military would never "sit idly by" if she were to visit Taiwan, the self-ruled island claimed by Beijing.
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.
Chinese foreign ministry spokesman Zhao Lijian said 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 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 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.
During a phone call last Thursday, President Chinese Xi Jinping warned his U.S. counterpart 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.
On Monday, Taiwan Premier Su Tseng-chang did not directly respond when asked whether Pelosi will visit on Thursday, as
local media have speculated.
"We always warmly welcome visits to our country by distinguished foreign guests," he told reporters in Taipei.
Pelosi was expected to arrive in Singapore on Monday for a two-day visit, broadcaster CNA reported, citing the country's
foreign ministry. The American Chamber of Commerce in Singapore was scheduled to host a reception with her on Monday afternoon, its website said.
On Sunday, Chinese air force spokesman Shen Jinke was quoted by state media as saying that Beijing would "resolutely
safeguard national sovereignty and territorial integrity".
Shen said at a military airshow that the air force has many types of fighter jets capable of circling "the precious island
of our motherland", referring to Taiwan.
Beijing considers Taiwan to be part of its territory and has never renounced using force to bring the island under its control.
Pelosi's Asian tour comes at a politically sensitive time for Chinese and U.S. leaders.
Xi is expected to secure a precedent-breaking third leadership term later this year during a once-in-five-years congress of the ruling Communist Party. In the United States, Biden's Democratic Party faces a hard fight to retain control of the House of Representatives at November's midterm elections.
Last Wednesday, Biden told reporters he thought the U.S. military believed a Pelosi visit to Taiwan was "not a good idea
right now".
