The foreign secretary has said he believes Osama bin Laden is still alive.

Jack Straw claimed on Monday that the number one target in the war on terrorism was probably still active as the head of the al Qaeda network, despite extensive international efforts to track him down.

He was speaking after the al-Jazeera Arabic television network broadcast an audiotape alleged to have been made by the man responsible for the September 11 attacks on New York and Washington.

The tape called on Muslims to rise up against American forces in Iraq and the Middle East.

Straw said it was safe to assume that the voice heard was that of bin Laden, although he had no concrete evidence.

"As far as we know, Osama bin Laden is still alive," he told the BBC's Today programme.

Searching for the terrorist leader was still a "key issue" for UK and US forces and agencies, the foreign secretary added, despite the distraction of the war in Iraq.

Straw confirmed that British troops were likely to be in Iraq for years to come, as the prime minister had indicated during his flying visit to the country on Sunday.

He said he could not give an "exact timescale" for their withdrawal but "it is not going to be months".

"I can't say whether it is going to be 2006, 2007," he added.

But there was no reason to believe power would not be handed over to Iraqis as planned on July 1, he insisted.

Remaining troops would be used for peacekeeping and training exercises, Straw said.