Thousands of U.S. Marines stormed deep into Taliban territory in an Afghan river valley on Thursday, launching the biggest military offensive of Barack Obama's presidency.

The Marines say Operation Khanjar, or Strike of the Sword, will be decisive and is intended to seize virtually the entire lower Helmand River valley, the heartland of the Taliban insurgency and the world's biggest opium poppy producing region.

In swiftly seizing the valley and holding ground there, U.S. commanders hope to accomplish within hours what overstretched NATO troops had failed to achieve over several years, and help secure Afghanistan for an August 20 presidential election after years of stalemate.

The intent is to go big, go strong and go fast, and by doing so we are going to save lives on both sides, Brigadier-General Larry Nicholson, commander of the Marines in southern Afghanistan, told his staff before the operation.

Violence in the Taliban-led insurgency is at its highest since the Taliban's ouster in 2001. The operation marks the first big test of Washington's new regional strategy to defeat the Taliban and its allies and stabilize Afghanistan.

With new tactics to win over the Afghan population and new commanders in place, the U.S. military is hoping to turn the tide of a war some in Washington have admitted they are not winning.

The U.S. military said later on Thursday that a soldier had been missing in southeastern Afghanistan since Tuesday, before the operation in Helmand began, and was thought to have been captured by militants. The Pentagon confirmed the incident.

A Taliban commander, Mullah Sangeen, told Reuters by phone from an undisclosed location the soldier was taken as a patrol walked out of its base in Paktika province and would only be released when the U.S. military freed Taliban fighters it held.

THOUSANDS OF FIGHTERS

The Taliban has vowed that its thousands of fighters in southern Helmand and Kandahar would fight back against the offensive. Only minor skirmishes were reported on the first day.

Thousands of Taliban mujahideen are ready to fight against U.S. troops in the operation in Helmand province, Mullah Hayat Khan, a senior Afghan Taliban commander, told Reuters in Pakistan by telephone from an undisclosed location.

In Islamabad, the Pakistan military said it was redeploying some of its border forces to block any Taliban fighters trying to flee the new offensive. Helmand shares a 200-km (130 mile) desert border with Pakistan's southwestern Baluchistan province.

The offensive came as the commander of foreign forces in Afghanistan, General Stanley McChrystal, held talks in Rawalpindi with Pakistani army chief General Ashfaq Kayani, a Pakistani military official said. He did not give any details.

The U.S. military said it had suffered no serious casualties in the early stages of the assault.

The Taliban said in a later statement one of their fighters had been killed and two wounded. Quoting spokesman Qari Mohammad Yousuf, it said 11 foreign troops were killed and wounded.

Britain's Ministry of Defense said in a statement two British soldiers were killed in an explosion in central Helmand on Wednesday in a related operation preceding Khanjar.

In southeastern Zabul province, Afghan police killed nine Taliban fighters and discovered a ton of explosives on Wednesday, the Interior Ministry said in a statement.

WAVES OF HELICOPTERS

Waves of helicopters landed Marines in the early morning darkness throughout the valley, a crescent of opium poppy and wheat fields criss-crossed by canals and dotted with mud-brick homes. Entrenched fighters defied NATO forces there for years.

Marines also dismounted from armored convoys before dawn and fanned out into the fields alongside the river as the sun rose.

About 4,000 Marines surged forward and thousands more were mobilized to assist them in one of the biggest operations by foreign troops in Afghanistan since the 1989 Soviet withdrawal.

The 10,000 Marines in Helmand Province, 8,500 of whom arrived in the past two months, form the biggest wave of an escalation ordered by Obama. The U.S. president has declared the Taliban insurgency in Afghanistan and neighboring Pakistan to be America's main foreign threat.

Large areas of Helmand have been outside government control. It produces the biggest share of Afghanistan's opium crop, which accounts for 90 percent of the world's heroin.

Launching such a bold operation carries great risk. A protracted, bloody fight could erode support for the war in the United States, among its NATO allies and Afghans.

Taliban fighters have had years to reinforce positions among the valley's irrigation ditches and canals but U.S. and NATO commanders hope a rapid, decisive victory in Helmand will prove the tipping point of the war.

We're going to seize the population from the Taliban and never let them go, Marine Lieutenant-Colonel Christian Cabaniss told his troops before they set out in armored convoys.

(Additional reporting by Sayed Salahuddin, Golnar Motevalli and Jonathon Burch in Kabul and Saeed Ali Achakzai in Pakistan; Writing by Paul Tait; Editing by Sanjeev Miglani)

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