As the US launches a major offensive in Helmand where the Taliban insurgency is at its fiercest, the BBC's Caroline Wyatt assess the tactics and dangers involved in such an operation.

    The US Marines say Operation Khanjar, or Strike of the Sword, will be decisive.

    It is intended to seize almost the entire lower Helmand River valley, which remains the heartland of the Taliban insurgency in Helmand and a major poppy-producing area for the production of opium, which helps fund the insurgency.

    US commanders hope this offensive will help turn the tide in the current stalemate against the Taliban. The aim of this major operation is to "clear, hold and build" in areas under Taliban influence ahead of Afghanistan's presidential elections this August.

    They want to stabilise the province and offer enough long-term security to allow the Afghans to build up their own civil governance, while Isaf forces train the Afghan police and Afghan National Army who will eventually take over full responsibility for ensuring Afghanistan's security.

    The US Marines are part of the 10,000 - 12,000 US forces flooding into southern Afghanistan, with a total of 17,000 extra American troops sent into Afghanistan by the new US administration.

    Re-energised campaign

    Their tactics are likely to be similar to those used in British military operations in Helmand, but on a far larger scale. The UK currently has around 9,000 forces in Helmand.

    The new US commander of Isaf, former special operations chief General Stanley McChrystal, has told commanders that he wants a shift away from conventional operations to counter-insurgency aimed at winning over Afghan civilians.

    His mantra is to "protect the people" - civilian casualties caused by Isaf military operations seen as one of the most counter-productive aspects of the campaign so far.

    Isaf staff in Kabul say that General McChrystal has already re- energised the campaign, bringing a new sense of focus across the 42 Nato allies' military, diplomatic and economic efforts in Afghanistan.

    The hope is also that if US and other Nato forces are seen as clearly winning the military battle against the insurgents, middle and lower- ranking Taliban leaders and fighters are more likely to defect back to the Afghan government's side, with American and British sources in Kabul convinced that the US troop surge is already unsettling the Taliban leadership.

    Explosives Danger

    This American offensive follows smaller British offensive operations further north in Helmand Province, including Panther's Claw, which took back control of Babaji and the surrounding areas north of Lashkar Gah from the Taliban.

    Around 350 troops from the Black Watch, the 3rd Battalion The Royal Regiment of Scotland, launched the attack on 19 June in one of the UK military's biggest co-ordinated air operations of modern times.

    One of the main dangers to US and UK forces remains the Taliban's increasing use of improvised explosive devices (IED) - the cause of most of the recent British fatalities.

    While detection rates are improving - British sources say their forces are now finding over half the IEDs before they are laid - they remain difficult to combat.

    The Taliban are also increasing the amount of explosives used in their roadside bombs as British forces switch to more heavily-armoured vehicles.