Hundreds of British troops have key seized canal crossings in a Taliban stronghold in southern Afghanistan, military officials said on Friday, part of a new U.S.-led operation to wrest the initiative from insurgents.

The British push, one of the largest its overstretched troops have made in the Taliban heartland and key opium-producing province of Helmand, is part of a wider offensive launched by thousands of U.S. Marines on Thursday.

The Marines met little resistance on the first day of Operation Khanjar, or Strike of the Sword, the first big test of U.S. President Barack Obama's new regional strategy to defeat the Taliban and its allies and stabilise Afghanistan.

Their objective is to seize virtually all of the lower Helmand River valley, the world's biggest opium poppy-producing region, and hold the ground they win, something British-led NATO troops have so far been unable to do. (For a factbox on opium and the insurgency, click on)

Violence in the Taliban-led insurgency is at its highest since the Taliban's ouster in 2001 and the offensive in the short-term at least is meant to provide a secure environment for Aug. 20 presidential elections.

In the longer term, U.S. and NATO troops want to engage with local populations as part of a new counter-insurgency strategy under General Stanley McChrystal, appointed as the new commander of foreign troops in Afghanistan after previous conventional warfare tactics failed.

With new tactics to win over the Afghan population and new commanders in place, the U.S. military hopes the operation will mark the turning point of a war some in Washington have admitted they are not winning.

Hundreds of British soldiers have seized 13 canal crossings since Operation Panchai Palang, or Panther's Claw, began 10 days ago with an airborne assault north of the provincial capital Lashkar Gah. It is part of the overall Marine-led operation.

On Friday, another 800 British troops began pushing north towards Gereshk, Helmand's main industrial city.

"Taking control of the crossings will now allow British troops to prevent insurgents' movements between Helmand's two largest cities, Gereshk and Lashkar Gah, and will ultimately improve security and freedom of movement for the local people," the British military said in a statement.

SCATTERED CLASHES

Large areas of Helmand have been outside government control for many years. It produces more than half of Afghanistan's opium crop, which accounts for 90 percent of the world's heroin. The opium trade is a major source of funding for the Taliban.

Only scattered clashes were reported on Friday as the Marines fanned out through towns and mud-brick villages in the Helmand River valley, a crescent of opium poppy and wheat fields criss-crossed by canals.

The Marines were accompanied by "civilian stability advisers" and were meeting local community leaders, spokesman Captain Bill Pelletier said.

"What this is doing is starting to transition from the clearing operation to the holding part, our Marines are going to stay there and continue to provide security," he said.

One Marine was killed and several wounded on Thursday. No reports of civilian casualties were received, while the Taliban said one of its fighters had been killed.

Pelletier said most resistance so far consisted of groups of two or three insurgent fighters.

"As soon as our resources were brought to bear on them they would break contact and run away," he said. "We're not taking anything for granted, the enemy will resist."

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

Insurgents had launched a series of attacks against Welsh Guards troops during their operation to seize the canal crossings, the British military said, and almost 100 roadside bombs were found and made safe since it began.