Chinese president Hu Jintao has phoned President George W Bush to warn the US against selling more military technology to Taiwan.
President Hu made surprisingly candid remarks
Washington is negotiating a deal to sell missiles and other weapons systems worth $18bn to the island - which China regards as a renegade province.
Mr Hu said Beijing would do its utmost to resolve the Taiwan issue peacefully.
But he said China would never tolerate the island's independence, or allow anyone to split it from China.
Spokesmen in Washington and Beijing said Mr Bush reaffirmed his backing for the one-China policy - which does not support Taiwanese independence - while reasserting America's commitment to help Taiwan defend itself.
The BBC's Rupert Wingfield-Hayes in Beijing says that the fact that President Hu took up the Taiwan issue in a personal telephone call to Mr Bush shows just how strongly China feels about it.
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
On Friday China's vice-minister for Taiwan, Wang Zaixi, warned that Beijing might attack Taiwan by 2008 if its pro-independence President Chen Shui-bian pursued plans for constitutional change.
Tensions between Taiwan and China have been rising ever since Mr Chen was re-elected in March.
Now he wants to spend $18 billion to buy advanced missiles, aircraft and submarines from the US.
President Bush says he opposes Taiwan independence.
But from China's point of view, his willingness to sell the island such advanced weapons is sending a very different message, our correspondent says.