The expected appearance of US helicopter gunships above Afghanistan over the next few days would signal the start of a new and deadly stage of the American-led offensive against the Taleban. The Blackhawk helicopters are likely to be the only visible sign of commando-style attacks by US special forces and British SAS troops - ground raids that risk the first allied casualties. Nine days of bombing had destroyed Taleban air defences and troop concentrations but even covert ground missions remained highly dangerous, Pentagon sources warned. The troops have reportedly been active inside Afghanistan for several weeks preparing for the ground attacks. With conventional defences destroyed, US military analysts believe they now face a guerilla-style opposition that could take months to eradicate. The enemy includes the Taleban's elite 55th brigade, funded by Osama bin Laden, and an estimated 3,000 fighters loyal to his al-Qaeda network. The 55th brigade has been strengthened over the past year with foreign fighters from across the Arab world. It is thought to be 1,000 strong. A network of mountain caves - defensive redoubts for centuries in rugged Afghanistan - has proved impenetrable to US surveillance techniques and to those of Soviet forces before them. The biggest unknown is the amount of weaponry the Taleban and al-Qaeda have been able to hide from attack. The Taleban has some mobile missiles and artillery on trucks that have yet to be found, but of most concern are al-Qaeda's anti-helicopter weapons. US intelligence sources said the group was thought to still possess rocket-propelled grenades and Soviet-era SA-7 and US Stinger missiles, relics of Moscow's failed occupation. Both can be fired from the shoulder, lock on to heat sources and have a range of up to 3,000 metres - making helicopters vulnerable. The CIA supplied anti-Soviet mujahedeen rebels - some who now fight with bin Laden - with 1,000 Stingers during the Cold War and about 400 are thought to remain inside Afghanistan. 'There are definite fatal risks attached to the next phase,' one senior Pentagon official said. 'It might not be clean and it might not be swift.'