Chinese President Xi Jinping has urged the people of Taiwan to accept it "must and will be" reunited with China.
In a speech marking 40 years since the start of improving ties, he reiterated Beijing's call for peaceful unification on a one-country-two-systems basis.
However, he also warned that China reserved the right to use force.
While Taiwan is self-governed and de facto independent, it has never formally declared independence from the mainland.
Beijing considers the island to be a breakaway province and Mr Xi's comments are in line with China's long-standing policy towards reunification.
But on Wednesday, Taiwan's President Tsai Ing-wen said the island would never accept reunification with China under the terms offered by Beijing.
"I want to reiterate that Taiwan will never accept 'one country, two systems'. The vast majority of Taiwanese public opinion also resolutely opposes 'one country, two systems', and this is also the 'Taiwan consensus'."
Under the "one country, two systems" formula, Taiwan would have the right to run its own affairs; a similar arrangement is used in Hong Kong.
Hong Kong has its own legal system, and rights including freedom of assembly and free speech are protected - however, there are widespread concerns in the territory that those freedoms are gradually being eroded.