Almost 4,000 US forces joined 650 Afghan police and soldiers in the pre-dawn launch into southern Helmand province.

As the surge got under way, a US military spokeswoman confirmed an American soldier was taken prisoner by insurgents in eastern Afghanistan.

Captain Elizabeth Mathias said the soldier has been missing since June 30.

Meanwhile, Pakistani and US officials were concerned the offensive in southern Afghanistan could push militants across the poorly-guarded, mountainous border into Pakistan.

"We've mustered more troops from the other areas of the border" to position opposite the Helmand region, said Pakistan's Major General Athar Abbas.

Operation Khanjar (Strike of the Sword) deployed 50 aircraft, as US President Barack Obama's new war plan swung into action.

Officers said the aim of the air and land assault was to push troops into insurgent strongholds.

"What makes Operation Khanjar different from those that have occurred before is the massive size of the force introduced, the speed at which it will insert," US Brigadier General Larry Nicholson said.

American troops would hold taken areas until they could transfer security responsibilities to Afghan forces, he added.

Military commanders said Operation Khanjar would convince local people that the Afghan security forces - backed by international troops - offered them a better long-term future than the Islamist hardliners.

Set to last 36 hours in its first highly aggressive phase, the offensive aims to bring security to the Helmand valley ahead of presidential elections on August 20.

At a pre-launch briefing, Brig Gen Nicholson told his men: "This is a big, risky plan. It involves great risks and amazing opportunities. These are days of immense change for Helmand province.

"We're going down there, and we're going to stay - that's what is different this time."

Reflecting the new US strategy, he stressed that the security needs of Helmand's residents came before killing Taliban.

Key targets of the assault include the districts of Garmsir and Nawa, which are towards the southern border with Pakistan.

Many of the insurgents are said to take refuge in those locations, and are involved in the opium trade funding the Taliban.