Nancy Pelosi, the speaker of the U.S. House of Representatives, began a closely-followed Asia tour on Monday in Singapore as China threatened that its military would never “sit idly by” if she went to Taiwan, the self-ruled island claimed by Beijing.

Pelosi’s office said on Sunday that she was heading a Congressional delegation to the region, which would also include stops in Singapore, Malaysia, South Korea, and Japan, amid extensive rumors about whether she would make a stop in Taiwan. Taiwan wasn’t mentioned in it.

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.

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 third 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.

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.

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”.