The military base at Ben Guerir, some 60km (37 miles) north of the Moroccan city of Marrakech, is one of three emergency landing sites maintained by Nasa.

James Halsell, the shuttle manager, told reporters that Nasa had withdrawn from the north African site "in line with the worldwide elevation of security concerns".

He declined to elaborate.

Travel warning

On Tuesday, the US State Department urged US citizens in Morocco to take precautions against possible terrorist attacks on American targets there.

Mr Bin Laden is wanted in the US

It said the government had information that attacks might be planned, but did not know if the intelligence was credible.

US authorities issued a worldwide security alert last month after Saudi-born militant Osama Bin Laden threatened attacks on US targets.

The US accuses Mr Bin Laden of masterminding the 1998 attacks on its embassies in Kenya and Tanzania, that killed more than 200 people.

Security breach

Nasa's fears were further heightened when a group of illegal immigrants was found wandering near the launch site at Cape Canaveral in Florida.

Sixteen men were arrested and handed over to the immigration authorities last Friday.

The mission is delivering an airlock

They might have washed ashore there by coincidence, officials said, but the event underlined how little security Nasa has at the site.

Nasa has for a long time been asking for an increase in its security budget.

Thursday's Atlantis flight is to deliver an airlock to the ISS.

The $164 million piece of equipment is designed to work with both Russian and US spacewalking suits and will complete the first phase of ISS assembly.

Four men and one woman make up the Atlantis shuttle crew.