US House of Representatives Speaker Nancy Pelosi starts a tour of four Asian countries on Sunday, her office said, without mentioning Taiwan amid intense speculation she might visit the self-ruled island claimed by China.

"Speaker Nancy Pelosi is leading a Congressional delegation to the Indo-Pacific region, including visits to Singapore, Malaysia, South Korea and Japan," her office said in a press release.

The release said the visit would include those countries, but did not specify whether Pelosi, who is number 3 in the line of presidential succession, might make other stops.

"The trip will focus on mutual security, economic partnership and democratic governance in the Indo-Pacific region," it said.

Gregory Meeks, chair of the House Foreign Affairs Committee, was among the delegation listed.

China views visits by US 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 would be a dramatic, though not unprecedented, show of US support for Taiwan. Republican Newt Gingrich was the last House speaker to visit Taiwan in 1997.

President Xi Jinping warned his US counterpart Joe Biden on Thursday that Washington should abide by the one-China principle and "those who play with fire will perish by it".

Biden told Xi that US 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.

Taiwan's foreign ministry said on Friday after the call between Xi and Biden that Taiwan would continue to deepen its close security partnership with the United States.

Chinese air force spokesman Shen Jinke was quoted by state media as saying on Sunday 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.

He said China's "air force has the firm will, full confidence and sufficient capability to defend national sovereignty and territorial integrity."

A comment by a People's Liberation Army unit on Weibo, China's Twitter-like social media, on Friday — "Prepare for war!" — received 1.87 million thumbs-ups.

South China Sea

As a US aircraft carrier USS Ronald Reagan and its strike group returned to the South China Sea by Thursday, Chinese military ramped up exercises in the vicinity.

On Saturday, Chinese military held live-firing drills in the waters off Fujian province, over a 100 kilometres (62 miles) away from Taiwan, according to local authorities.

Chinese coast guard will hold an exercise in the South China Sea off Guangzhou province on Monday, according to another notice by the Maritime Safety Administration.

Prominent Chinese commentator Hu Xijin said on Saturday he deleted a tweet warning of military retaliation should US fighter jets escort Pelosi on a Taiwan visit, after Twitter blocked his account.

White House national security spokesperson John Kirby said on Friday the United States has seen no evidence of looming Chinese military activity against Taiwan.

On Wednesday, Biden told reporters he thought the US military believed a Pelosi visit to Taiwan was "not a good idea right now".

Pelosi's Asian tour comes at a politically sensitive time for Chinese and US leaders.

Xi is expected to seek a precedent-breaking third term at a Congress later this year, while in the United States, Biden's Democratic Party will face a hard fight to retain control of the US House of Representatives at November's midterm elections.