British troops have begun their biggest offensive since the start of the Afghanistan war, as US-led forces escalate their assault on al-Qaeda and Taliban fighters.

About 1,000 British troops were last night deployed in south-eastern Afghanistan as part of an operation expected to last some days. The US has meanwhile sent hundreds of troops and attack helicopters to the Afghan mountains, near Khost, on the border with Pakistan.

American officials said the two-pronged attack aimed to rout al-Qaeda terrorists and Taliban fighters, feared to be regrouping near the border with Pakistan.

Some intelligence reports have also suggested that Osama bin Laden, the al-Qaeda leader blamed by the US for the September 11 atrocities, may be hiding out on the Pakistani side of the border.

The British mission, using troops from 45 Commando Brigade, has been planned for several weeks, and is likely to dwarf in scale last month's Operation Ptarmigan - a sweep of the mountains in the south-east of the country.

The Americans are concerned that al-Qaeda have moved into Pakistan, and that, when the snow melts, they will return to Afghanistan to defeat the interim Afghan administration led by Hamid Karzai.

Brigadier Roger Lane, the UK force commander, would not specify the location of the mission, but said it was "one of the few remaining areas that has not been investigated by coalition ground forces", adding it was "not without risk . . . The area we are going to is a very difficult one in very difficult terrain". He said he had reason to believe "it is, or has been, a key base for the al-Qaeda network".

Donald Rumsfeld, US secretary of defence, said: "It's a situation where we know there are al-Qaeda and Taliban who in some instances have not left the country and if they have left the country they have not left very far."

US troops from the 101st Airborne Division will provide support to British marines already in the Khost area. A total of 7,000 US troops and 5,000 soldiers from allies are ready to strike.

A pool of 1,700 British troops, including Royal Marines, Royal Engineers, Royal Artillery and logistics support, were sent out to Afghanistan for Operation Ptarmigan - the UK's first major offensive since the start of the Afghan war in October. The British marines and US and Afghan forces united to clear out Taliban and al-Qaeda fighters in eastern Afghanistan, near Khost. Marines comb wilderness, Page 2 Pakistan referendum, Page 12 www.ft.com/terror

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