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.
The private warnings suggested a possible military response, the FT report said.
"We are seriously prepared," spokesman Zhao Lijian told reporters on Monday.
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."
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.
Meanwhile, Taiwan kicked off its largest annual military exercises Monday, with trench warfare and shoulder-launched Stinger missiles deployed against simulated Chinese attacks in drills, AFP reported.
China has time and again said the island is part of its territory and must rejoin the mainland even by force.