More than 200 police officers and soldiers are at the scene

City police said they had received a tip suggesting that extremists were planning a suicide car bomb attack.

The US airbase at Rhein-Main is also thought to be another possible target.

Several streets around the hospital in the suburb of Wansbeck have been sealed off and will remain so until further notice, police said.

'US warning'

Hamburg police said in their statement they received the information from unnamed "security sources" at 1430 local time (1330 GMT), AP reported.

Motassadek was convicted in Germany in relation to the 11 September attacks

German intelligence services later said the warning came from a US intelligence agency.

The threat is thought to come from the militant group Ansar al-Islam, based in northern Iraq.

Two members of the group were said to be planning to plant a car bomb outside the hospital, however it is not clear where the men are now.

German officials said the men, thought to be of Middle Eastern origin, travelled to the country in early December to carry out the suicide attack.

US officials have accused Ansar al-Islam of having links to Osama Bin Laden's al-Qaeda network.

More than 200 police officers armed with sub-machine guns, soldiers and explosives experts have been sent to the scene.

Police using sniffer dogs are also checking cars entering the area.

There has been speculation that US forces injured in Iraq are being treated at the facility.

The Hamburg connection

BBC correspondent Tristana Moore in Berlin says the threat is being taken very seriously by German authorities, who have already been on high alert over the Christmas period.

In February, a German court sentenced Moroccan Mounir al-Motassadek to 15 years in prison after he was convicted of helping three of the suicide pilots involved in the 11 September 2001 attacks.

The court which convicted Motassadek heard he was a member of the Hamburg cell which planned the terror attacks, which included pilots Mohammad Atta, Marwan al-Shehhi and Ziad Jarrah.