Olympic organisers vowed to tighten security in central Beijing [Images] on Sunday after a US tourist died in a stabbing attack but they insisted the Chinese capital was safe.

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"There is no reason to believe the attack was targeted on Americans," said Beijing Olympic spokesman Wang Wei.

Todd Bachman died in Saturday's attack by an unemployed Chinese man who then killed himself.

Assaults on foreigners in Beijing are rare. Police say they have no idea what the killer's motive was.

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"We are going to strengthen security in scenic spots... to make sure no sharp weapons can be brought into the area," Wang told reporters.

"Beijing is a safe city but unfortunately we are not immune to violent acts."

Bachman's wife, Barbara, was in a critical but stable condition on Sunday after undergoing eight hours of surgery. She suffered multiple lacerations and stab wounds in the attack.

The US Olympic Committee said members of her family, including her son-in-law Hugh McCutcheon, who is a coach with the US men's volleyball team for the Olympics [Images], were with her.

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The US men's volleyball team are playing their opening game against Venezuela on Sunday. McCutcheon, a New Zealander, will not be on the bench, US officials said.

Police said the tour guide, who was also injured in the attack, was out of danger.

Tang Yongming, 47, from the eastern city of Hangzhou, attacked the Bachmans and the Chinese tour guide shortly after midday at the centuries-old, 45-metre-high (148-ft-high) Drum Tower in central Beijing.

Tang, who had no previous criminal record, died after leaping from the Tower, police said.

"Because he is dead, there is no way to find out the motive," Wang said. All police knew was that he did not have a criminal record and had entered Beijing on Aug 1.

The US Embassy in Beijing said the attack "appears to be a senseless act of violence".

"This appears to be an isolated act with no connection to the Olympics," the embassy said in a statement. "We have no reason to believe that the assailant targeted the victims as American citizens."

US Ambassador Clark Randt visited the hospital on Saturday and conveyed President George W. Bush's condolences.