China is reported to have begun military exercises simulating an invasion of Taiwan.
By Francis Markus
BBC correspondent in Shanghai
The annual drills - involving 18,000 troops on the island of Dongshan - are intended as a further warning against moves towards formal independence by the self-governing island Beijing claims is part of China.
These are increasingly tense times between China and Taiwan.
China announced that the drills had begun through a Beijing-backed newspaper in Hong Kong.
They are an annual exercise, but the focus this year is reported to be on demonstrating Chinese air superiority, as Beijing strives to close the technology gap with Taiwan's US-supplied hardware.
In Washington itself, officials have held a crisis simulation looking at responses to the rising tensions.
Washington denies that either this or a new deployment of US aircraft carrier strike groups was aimed at anyone in particular.
But this year's election victory by Taiwan's pro-independence President Chen Shui-bian has left Beijing deeply worried by the drift towards a separate identity on the island.
And Chinese leaders could at some point be goaded into action by the voices in Taipei who say Beijing is bluffing when it threatens to retake Taiwan by force.