Jack Straw has warmly welcomed the fall of Kandahar and the end of Taliban rule in Afghanistan.

The foreign secretary described the final stages of the military operation in Afghanistan as a "remarkable achievement".

He refused to be drawn over the future of the movement's religious leader Mullah Omar and said it was not a "grave disappointment" that Osama Bin Laden had not been found at the cave complex of Tora Bora.

"It is a remarkable achievement within just four weeks of the fall of Kabul," Straw said. "The situation in Afghanistan and therefore the rest of the world has completely been transformed and in these four weeks the world has become a very much safer place and Afghanistan a very much happier place."

The foreign secretary said "it was always going to be difficult to find" Osama Bin Laden.

He said that the issue of whether Mullah Omar would be brought to trial was "entirely a matter" for America but stressed that the Taliban leader had not been allowed to walk free.

"You cannot let someone like Mullah Omar slip quietly into history," he said.

Conservative leader Iain Duncan Smith urged caution.

"Those who counselled against action have been proved to be wrong and Afghanistan will now benefit from the results. We should not, however, consider that the war is over. We still have to break the al-Qaeda network and Osama bin Laden and we must be prepared to continue the war against international terrorism wherever it raises its head," he said.