China has privately warned the United States about House Speaker Nancy Pelosi's visit to Taiwan, not ruling out a possible military response, according to the Financial Times.

The warnings were much harsher than those issued earlier when China was unhappy with US policies and actions related to Taipei, the newspaper said, citing unnamed sources.

Earlier this week, China's foreign ministry officially pledged a strong response to Pelosi's visit to Taiwan, which was reported to take place next month. Her trip will be the first trip by a speaker of the House of Representatives to Taipei since 1997.

President Joe Biden earlier also expressed doubts about whether Pelosi's visit would take place, saying the military thought it was "not a good idea."