A U.S. marine guards a road near the Taliban stronghold of Nawa in Afghanistan's Helmand province. (David Guttenfelder/Associated Press)

U.S. forces began to secure key centres in Afghanistan's Helmand province on Friday as a major offensive aimed at pushing out the Taliban entered its second day.

Nearly 4,000 U.S. marines and 650 Afghan forces moved into southern Afghanistan early Thursday under the cover of darkness as part of Operation Khanjar, also known as Strike of the Sword.

Transport helicopters carried marines into the village of Nawa, about 30 kilometres south of the provincial capital of Lashkar Gah, in a region where no U.S. or other NATO troops have previously operated in large numbers.

As the operation entered its second day on Friday, the units secured control of the district centres of Nawa and Garmser, and negotiated entry into Khan Neshin, the capital of Rig district, said marine spokesman Capt. Bill Pelletier.

Scattered clashes were being reported Friday as troops continued to fan out in the region and enter more towns and villages. The largest engagement between U.S. troops and militants reportedly occurred near the town of Garmser.

1 killed

Though there have not yet been any big battles, one marine was killed Thursday and several were wounded, U.S. military officials said. No further details were provided.

Resistance has come mostly from small groups of two or three militants, Pelletier said.

U.S. marines climb onto the roof of a house near Nawa. (David Guttenfelder/Associated Press)Officials said most of the Taliban militants that have been encountered so far have retreated from U.S. forces rather than engage in battle.

There have been no reports of civilian or militant causalities.

Pelletier said the unit is preparing for change in the militants' tactics in the coming days. But the operation's focus is not killing the Taliban but winning the local population over, he said.

"It is important to engage with the key leaders, hear what they need most and what are their priorities," Pelletier said.

U.S. forces are already speaking with the elders in the communities that have been secured, he said.

Officials on the ground reported that residents in the region were expressing concern that troops would enter homes without permission or restrict their religious practices.

There were also reports that some residents were expressing fear about what would happen to civilians if the offensive were unsuccessful and the militants were allowed to come back into the region, officials said.

Locals weary of interference

Lashkar resident Haji Akhtar Mohammad told The Associated Press that the population in Helmand province is weary of foreign interference and the U.S. forces may have difficulty garnering community support.

He added that identifying militants versus civilians in the area might also prove difficult.

"It is difficult to tell who is Taliban and who is civilians," Mohammad said. "They all have the same face, same beard and same turban."

The operation along a 90-kilometre section of a river valley in the poppy-growing region of Helmand province is the first under U.S. President Barack Obama's strategy to stabilize Afghanistan and is aimed at removing insurgents ahead of the country's Aug. 20 presidential election.

The insurgency has proven particularly resilient in the area, and foreign troops have never before operated there in such large numbers.

Thousands of British forces, fighting under NATO command, have been in Helmand since 2006 with broadly the same strategy, but security has deteriorated. They have met with stronger resistance than initially expected against Taliban fighters bankrolled by the vast opium and heroin trade.

The Obama administration has made the Afghan mission a top military priority, boosting American troop numbers in Afghanistan by 21,000.

Russia will allow the U.S. to ship weapons across its territory to Afghanistan for the offensive, the Kremlin announced Friday. It is unclear if personnel will be allowed to travel through Russian territory or airspace.

The deal is expected to be signed during Obama's visit to Moscow next week.

With files from The Associated Press