Speaking at Beijing’s Great Hall of the People on the 40th anniversary of a key Taiwan policy statement, Xi said reunification must come under a one-China principle that accepts Taiwan as part of China, anathema to supporters of Taiwan independence.
All people in Taiwan must "clearly recognise that Taiwan independence would only bring profound disaster to Taiwan", Xi said.
"We are willing to create broad space for peaceful reunification, but will leave no room for any form of separatist activities," he said.
"We make no promise to renounce the use of force and reserve the option of taking all necessary means," he said, adding that the issue is China’s internal affair and that it would permit "no external interference".
Taiwan is China’s most sensitive issue and is claimed by Beijing as its sacred territory. Xi has stepped up pressure on the democratically governed island since Tsai Ing-wen from the pro-independence Democratic Progressive party became president in 2016.