Washington cannot stand by while the Chinese Communist Party threatens Taiwan and democracy itself, US House Speaker Nancy Pelosi said.

Pelosi arrived in Taiwan despite protests from Beijing on Tuesday. This is the first visit by a speaker of the US House of Representatives to the island since 1997.

"In the face of the Chinese Communist Party's (CCP) accelerating aggression, our congressional delegation's visit should be seen as an unequivocal statement that America stands with Taiwan," Pelosi tweeted.