He was responding to criticism that Operation Snipe has been a failure.

There is a sense of frustration and there is no doubt about that

Lieutenant-Colonel Ben Curry

Sixteen days after leaving Bagram air base, 1000 British troops have not captured anyone.

But Mr Blair refused to confirm or deny an allegation that they blew up an ammunition dump belonging to Britain and America's Afghan allies.

He said that this was a logistical matter for the Ministry of Defence.

But Mr Blair said that the marines had harried and undermined al-Qaeda forces and found and destroyed munitions clearly belonging to them.

"The al-Qaeda has been wiped out effectively in Afghanistan," he said, adding that British troops were fulfilling a very important purpose.

Anger

Royal Marines Brigadier Roger Lane, who is in charge of the British troops in Afghanistan, is understood to be angry at the criticism being levelled by newspapers and politicians at home.

But his spokesman, Lieutenant-Colonel Ben Curry, admitted that there was a "sense of deflation" on the part of British troops.

Blair said the Royal Marines served an important purpose

"There is a sense of frustration and there is no doubt about that.

"We came out here to close with and kill al-Qaeda and Taleban. We've not been able to do that - yet," he told the BBC.

He said these were still "early days".

He was speaking after the Mirror newspaper featured graffiti scrawled on a toilet wall at the Bagram airbase featuring marines voicing their discontent over the operation.

Weapons cache

The Royal Marines have been accused of blowing up an arms cache belonging to a coalition ally, rather than al-Qaeda forces.

The weapons were blown up at the weekend as part of Operation Snipe and said to belong to al-Qaeda forces in the Paktia province.

The operation was initially hailed as a success after four caves stacked with ammunition - 20 truck loads of weaponry - were discovered.

The weapons' destruction was described as the biggest explosion of its kind since World War II and a major blow to enemy forces.