The report cited six people familiar with the Chinese warnings as saying they were significantly stronger than the threats that Beijing has made in the past when it was unhappy with US actions or policy on Taiwan, which is claimed by China.
The private rhetoric suggested a possible military response, the Financial Times cited several people familiar with the situation as saying.
The White House National Security Council and the State Department declined to comment on the report. China's foreign ministry did not immediately respond to a Reuters request for comment on Sunday.
China has been stepping up military activity around Taiwan seeking to pressure the government there to accept Chinese sovereignty. Taiwan's government says only the island's 23 million people can decide their future, and while it wants peace will defend itself if attacked.
The Financial Times reported on Monday that Pelosi plans to visit Taiwan in August.
China's foreign ministry said the next day a visit to Taiwan by her would seriously undermine China's sovereignty and territorial integrity, and the United States would bear the consequences of its response.
On Wednesday, US President Joe Biden said he plans to speak with Chinese President Xi Jinping by the end of the month. Biden appeared to cast doubt on the reported Pelosi trip to Taiwan.
"I think that the military thinks it's not a good idea right now, but I don't know what the status of it is," Biden told reporters.