China reserves the right to use force to bring Taiwan under its control, but will strive to achieve peaceful "reunification", President Xi Jinping has said.
The self-ruled island of Taiwan is China's most sensitive issue and is claimed by Beijing as its sacred territory.
Mr Xi has stepped up pressure on the proudly democratic island since Tsai Ing-wen from the pro-independence Democratic Progressive Party became president in 2016.
The Chinese leader has set great personal store in resolving what the Communist Party calls the "Taiwan issue", holding a landmark meeting with then Taiwan president Ma Ying-jeou in Singapore in late 2015, just before Ms Tsai was elected.
Mr Xi spoke at Beijing's Great Hall of the People on the 40th anniversary of a key Taiwan policy statement.
He said "reunification" must come under a one-China principle that accepts Taiwan as part of China, anathema to supporters of Taiwanese independence.
The vast majority of Taiwan's people are clearly aware that independence would lead to a "grave disaster", Mr Xi told an audience that included Taiwanese business people and senior party officials.
"China won't attack Chinese people. We are willing to use the greatest sincerity and expend the greatest hard work to strive for the prospect of peaceful reunification," Mr Xi said.
"We do not promise to renounce the use of force and reserve the option to use all necessary measures" to achieve this goal and prevent Taiwan independence, he said.
This, though, was aimed at foreign forces who seek to interfere and the tiny minority of Taiwan independence forces and their activities, Mr Xi said without elaborating in what was likely a reference to the United States, Taiwan's strongest backer.
There was no immediate reaction from Taiwan's government.
Mr Xi reiterated that China was willing to talk with any party in Taiwan to push the political process - stalled by China since Ms Tsai took office - as long as they accept the "one China" principle.
He sought to reassure people in Taiwan that there was nothing to fear from Chinese rule, even though most people there have shown no interest in being run by autocratic Beijing.
"After peaceful reunification, Taiwan will have lasting peace and the people will enjoy good and prosperous lives. With the great motherland's support, Taiwan compatriots' welfare will be even better, their development space will be even greater," Mr Xi said.
Ms Tsai, who says she wants to maintain the status quo with China, said yesterday China must use peaceful means to resolve its differences with Taiwan and respect its democratic values.
China has regularly sent military aircraft and ships to circle the island on drills in the past few years and has heaped pressure on the island internationally, including whittling down its few remaining diplomatic allies.
Taiwan is gearing up for presidential elections in a year. Ms Tsai's party suffered stinging losses to the China-friendly Kuomintang in mayoral and local elections in November.
Mr Xi was speaking on the anniversary of the "Message to Compatriots in Taiwan" on 1 January 1979, when China declared an end to what had been routine artillery bombardment of Taiwan-controlled offshore islands and offered to open up communication between the two sides.
However, the offer was rebuffed by Taiwan's then-president Chiang Ching-kuo, who in April that year came out with a "three nos" policy of no contact, no compromise and no negotiation with China.