On Monday, U.S. House Speaker Nancy Pelosi began a carefully watched Asia tour in Singapore, as China warned that its military would never “sit idly by” if she visited Taiwan, the self-ruled island claimed by Beijing.

Following extensive discussion over whether she would visit Taiwan, Pelosi’s office said on Sunday that she would be heading a congressional delegation to the area, which would include stops in Singapore, Malaysia, South Korea, and Japan. Taiwan was not mentioned.

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.

Visits by US officials to Taiwan, according to China, send an encouraging signal to the island’s pro-independence side. Although Washington does not have formal diplomatic relations with Taiwan, it is required by law to assist the island with the tools to defend itself.

A visit by Pelosi, who is second in line to the presidency and a long-time critic of China, would occur amid deteriorating relations between Washington and Beijing. The last House speaker to visit Taiwan was Republican Newt Gingrich in 1997.