UK forces prepare for biggest Afghan offensive since 2001

15,000 troops set to launch major assaults on Taliban strongholds in Helmand

The bloodiest fighting in the eight-year war in Afghanistan is expected to break out this week as British troops yesterday continued to prepare for what will be the largest air assault seen since the first Gulf war in 1991.

Military experts said the fighting was "imminent" after several days of so-called "shaping operations" in the Nad Ali area of Helmand and around the town of Marjah, which is seen as a "hub" of Taliban activity and a poppy-growing centre. "Shaping" is the moving of troops into position, with a cordon likely to be thrown up around Marjah, a town of about 80,000 people south-west of Lashkar Ghar.

The offensive is expected to see heavy casualties on both sides, with civilian casualties "inevitable". Colonel Richard Kemp, who commanded forces in Afghanistan in 2006, said the Taliban would embed themselves among the population and are also using children to lay IEDs (improvised explosive devices) and to throw grenades in order to avoid snipers.

"There will be some heavy fighting," he said. "The Taliban will have carefully placed their positions among the population centres. They will know the area very well and will have prepared escape routes through tunnels, alleyways or buildings. They will fire on our troops and then run. There will be a lot of IEDs, a lot of snipers and a lot of hit and run. We will probably have to brace ourselves for a large number of casualties. They are increasingly using children to move IEDs and to throw grenades."

The 15,000 troops involved in Operation Moshtarak – which means "together" in Dari – is five times the number involved in last year's Operation Panther's Claw, which had until now been the largest offensive in Afghanistan. Colonel Kemp added that the offensive, which includes at least 4,000 British troops, would see the largest movement of troops by air since the 1991 assault on Iraq, and would be on a similar scale.

The offensive is the first big test for President Obama's surge, with some of the 30,000 additional troops he sent to Afghanistan being deployed for Operation Moshtarak. Last May allied and Afghan troops launched a four-day operation against Marjah, where at least 1,000 Taliban soldiers are thought to be based. This resulted in the "record seizure" of 92 tons of heroin and is said to have "severely disrupted" a command hub of the insurgency.

That the town is once again to be attacked reveals the difficulties low troop numbers have had in maintaining security, said defence expert Paul Beaver. "We've had insufficient troops to hold the ground," he said. "We're now in a position where the Afghan army is better trained, so they should be able to take over."

Colonel Kemp added that the offensive, early in the fighting season which does not normally being until March, will open a summer of fighting as the Americans seeks to place a lid on Afghanistan before July next year, when President Obama pledged to start "drawing down" troops.

Operation Moshtarak is also a response to a terrible 2009, the deadliest year for British soldiers in Afghanistan, with 108 killed. "I don't think Britain or America took Afghanistan seriously until last year," Col Kemp said. "The level of our casualties and the resurgence of the Taliban meant that last year was the turning point in their thinking."

The Ministry of Defence said it was unable to comment on the specifics of the operation.