A senior Chinese official has warned that Beijing may attack Taiwan by 2008 if President Chen Shui-bian pursues his plans for constitutional change.
President Chen denies he is seeking Taiwan independence
Mr Chen has insisted the planned amendments are only concerned with local issues, but Beijing suspects he has a pro-independence agenda.
Wang Zaixi, vice-minister of the State Council's Taiwan Affairs Office, said military action could not be ruled out.
The unusually forthright comments came as both sides staged annual war games.
China sees Taiwan as a renegade province which it has threatened to reunite by force if Taiwan declared independence.
Mr Wang, in an interview with the Chinese state newspaper China Daily, said Mr Chen's plan to write a new constitution by 2006, and - subject to a referendum - enact it by 2008, was a serious threat to cross-Strait stability.
CHEN'S REFORM PLANS
President Chen wants a new constitution by 2008
Aims to modernise political institutions, increase efficiency, enshrine human rights and review role of National Assembly
China says plan is the 'biggest threat to peace and stability across the Taiwan Strait'
But Mr Chen insists the changes will not touch
"New tensions may arise and even a serious crisis in the cross-Straits situation, if Chen obstinately pursues his timetable," he said.
"We cannot completely rule out the possibility (of a military conflict) though it is not at all what we hope for." The parentheses were provided by the newspaper.
China regularly uses strong language to denounce Mr Chen, but it is less usual for senior officials to talk openly of the risks of conflict.
Analysts say the row over Taiwan's proposed constitutional changes is likely to heat up in coming months as Mr Chen moves forwards with his plans.
Mr Wang's comments are also significant because China hosts the Olympics in 2008, and analysts in Taiwan had assumed China would be reluctant to contemplate any military action before then.
Mr Wang went on to say that it was difficult to improve current cross-Strait relations unless President Chen embraced the "one-China" principle - that there is only one China, and Taiwan is part of it.
Both Taipei and Beijing have been conducting war games in the last two weeks, preparing for the eventuality of cross-Strait conflict.
President Chen took part in a submarine dive on Thursday, and China has staged a mock-invasion of the island.