Up to 15,000 troops - made up of British, American and Afghan soldiers - are ready to embark on Operation Moshtarak.

The battle promises to be a bloody one and defence chiefs have warned to expect more British casualties.

Sky's chief correspondent Stuart Ramsay has been with soldiers based in the Babaji area of Afghanistan as they make their way across highly exposed farmland in the area.

He said troops were often just metres away from the Taliban and were regularly hit by enemy fire.

Speaking in darkness from Helmand, Ramsay said: "The countdown clock for Operation Moshtarak has actually started - we just don't know when the clock will stop.

"But it is worth remembering that British, American and Afghan forces are already carrying out operations. They are testing the Taliban."

Ramsay said that soldiers were "flushing out" the enemy ahead of the fight, a phase known as "shaping".

"Soldiers are as close as 100 yards from the Taliban at any one time. It's extremely dangerous," he said.

Ramsay said it was "striking" how little control the British forces have in areas at the extreme end of the land taken in Operation Panther's Claw last year.

"And that is why Operation Moshtarak is being planned," he added.

"To take and hold towns and villages and land from the Taliban permanently - at least that is the plan."

Sky's foreign affairs editor Tim Marshall said Operation Moshtarak could mark the end of the middle phase of Nato's involvement in Afghanistan.

Around 4,000 British troops will be involved, together with up to 9,000 Americans and a large contingent of Afghan soldiers.

Moshtarak means "together" in Dari. The name is designed to signal that the Afghan Army is now playing an equal role in fighting the Taliban.

The assault focuses on the town of Marjah and will be the biggest test so far for the Afghan forces.

Marshall said: "Success could eventually lead to a significant reduction of troop numbers in an orderly fashion.

"Failure could lead to a rush for the exit. It will be months before the picture is clearer."