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 U.S. 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 has been stepping up military activity around Taiwan seeking to pressure the democratically-elected 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.
On July 18, the Financial Times reported that Pelosi plans to visit Taiwan in August.
