The FT report, published on Saturday, 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 FT cited several people familiar with the situation as saying.

The White House National Security Council and the State Department declined to comment on the FT report.

"We are seriously prepared," spokesman Zhao Lijian told reporters at a regular briefing on Monday, when asked for comment about the contents of the report.

When asked what kind of response China was "seriously prepared for" and if it would be a military or a diplomatic response Zhao said: "If the U.S. side is bent on going its own way, China will take strong measures to resolutely respond and counteract."

"The United States should be held responsible for any serious consequences," he added.

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.

The Financial Times reported last week that Pelosi plans to visit Taiwan in August. On Wednesday, U.S. 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.

• Biden: Military Thinks Pelosi's Trip to Taiwan 'Not a Good Idea'