China warned Wednesday that Washington would "bear the consequences" if US House Speaker Nancy Pelosi visits Taiwan, with tensions soaring between the two countries.

"We are firmly opposed to Speaker Pelosi's visit to Taiwan," Chinese foreign ministry spokesman Zhao Lijian said at a regular press conference.

"If the US pushes ahead and challenges China's bottom line... the US side will bear all the consequences arising therefrom," he added.

Democratic Taiwan lives under constant threat of being invaded by China, which views the self-ruled island as part of its territory to be seized by force if necessary.

A possible visit by Nancy Pelosi -- yet to be confirmed by the senior Democrat -- has stirred alarm in President Joe Biden's administration, which fears the trip may cross red lines for China.

It comes at an especially fraught time as President Xi Jinping, China's most powerful leader in decades, prepares to cement his rule later this year at a major party meeting.

Pelosi told reporters last week it was "important for us to show support for Taiwan," while denying Congress was pushing for independence for the island.

Taiwan enjoys bipartisan backing in divided Washington and China's warnings have only fuelled calls for Pelosi to go ahead.