Taiwan held widespread air raid drills on Monday as China doubled down on warnings to the U.S. against allowing House Speaker Nancy Pelosi to visit the island.

Pelosi has defended her planned trip to the island, a region that China has long claimed as its sovereign territory. The Chinese foreign ministry doubled down on warnings that the trip could have "serious consequences" for the U.S.

"The Chinese side has made it clear to the U.S. on many occasions that it is firmly opposed to Speaker Pelosi's visit to Taiwan. We are fully prepared," foreign ministry spokesman Zhao Lijian said Monday.

"If the U.S. goes its own way, China will certainly take firm and forceful measures to safeguard its national sovereignty and territorial integrity, and the United States should be held responsible for any serious consequences," he added.