Two SAS squadrons - a total of about 100 soldiers - have been operating on their own, without American minders, in rugged mountainous terrain over the past few months, the sources say.

But unlike operations involving American or Australian special forces or Royal Marine commandos - who began returning home yesterday - the discussion of SAS operations are covered by an official blanket ban by the Ministry of Defence.

Four SAS soldiers were wounded, one of them seriously, in an intense firefight during an assault on caves in the mountains near Kandahar last November which left 18 enemy dead. They have continued to scour the mountains in operations conducted without US support, according to defence officials, killing dozens more enemy fighters in a number of clashes.

The SAS prefer to operate alone. They regard themselves as fitter than their US counterparts, although they are envious of the better communications equipment which is available to the American special forces, including the Delta Force.

Sources confirm reports of frustration and rivalry between the two - notably last December when US commanders prevented the SAS from searching for al-Qaida fighters in the caves around Tora Bora, where it was believed that Osama bin Laden was hiding.

The Americans wanted US special forces to do the job. But by the time US commanders had discussed the risks involved and what air cover was needed, Bin Laden and his al-Qaida fighters had left, sources say. They say the SAS have been repeatedly frustrated by the practice of American commanders of referring operational decisions to command headquarters in Tampa, Florida, and Washington.

Well-placed sources report SAS engagements with groups of al-Qaida fighters who were well armed but appeared to be under the influence of drugs.

In one incident, after returning fire, the fighters waved their hands in the air shouting slogans before hiding behind rocky outposts awaiting another volley from the SAS.

It is impossible to obtain official confirmation of the accounts of SAS operations, because of the MoD's policy of never discussing the activities of special forces. However, the ministry is anticipating the prospect of a flood of disclosures when SAS troops return from Afghanistan and leave the force.

It is undertaking a review of the existing blanket ban, which some senior officials are advising will not hold.

Special forces are likely to be increasingly in demand in the fight against international terrorists, an unconventional enemy widely expected, after the experience of the Afghanistan campaign, to concentrate on guerrilla warfare.

The SAS, which is experienced in counter-insurgency operations, is already showing signs of overstretch, according to defence sources. But it is determined to resist moves floated by the defence secretary, Geoff Hoon, to increase its size from the present 400 troops. That would dilute the elite nature of the force, SAS commanders say.

The first batch of marines from 45 Commando arrived back at their base at Arbroath, Scotland, last night after three months in Afghanistan where, according to the MoD, they found and destroyed ammunition and weapons dumps and deterred al-Qaida terrorists from attacking friendly Afghan forces, but did not fire a shot in anger.

The marines were the first of 1,700 due to pull out of Afghanistan over the next two weeks after the end of Operation Jacana, Britain's largest combat deployment since the 1991 Gulf war.

"I'm looking forward to what everyone is looking forward to," marine Alan Hazelwood told Reuters news agency at Kabul airport as he waited to board an unmarked Boeing 747 heading for Scotland. "A cold beer out of a pint glass."

Mr Hoon told reporters at Bagram airbase north of Kabul on Tuesday: "We are ready to take part in further operations should that be necessary."