U.S. marines move in formation through farm fields on Thursday after landing by helicopter in an overnight air assault near the Taliban stronghold of Nawa in Afghanistan's Helmand province. (David Guttenfelder/Associated Press)

Most Taliban militants encountered by U.S. troops on the first day of a massive new offensive retreated rather than engage in battle, military officials said Thursday.

Nearly 4,000 U.S. marines and 650 Afghan forces moved into southern Afghanistan early Thursday under the cover of darkness as part of an operation called Khanjar, which translates 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 operated in large numbers.

There were reports of gunfire being exchanged, helicopters firing rockets and rocket-propelled grenades being launched from houses as the sun came up in the region.

But in the first day of the offensive U.S. troops did not suffer any serious injuries and resistance from militants was only sporadic, said unit spokesman Lt. Abe Sipe.

"The enemy has chosen to withdraw rather than engage for the most part," Sipe said.

Taken by surprise

The troops dropped behind Taliban lines took many insurgents by surprise, said Capt. Drew Schoenmaker.

U.S. soldiers walk beside a convoy of army trucks in Taliban-held territory in Afghanistan on Thursday. (Reuters)"We are kind of forging new ground here. We are going to a place nobody has been before," said Schoenmaker, 31, who commands Bravo Company of the 1st Battalion, 5th Marine Regiment.

The operation 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.

Pakistan moved troops along its Afghan border in an effort to stop militants from entering the country to escape a new U.S.-led offensive.

Pakistani Maj.-Gen. Athar Abbas said there has been visual confirmation that militants are fleeing into Pakistan but declined to provide further details.

'End game'

Pakistan and U.S. officials have expressed concerns American troop buildup in Afghanistan could push militants into the poorly guarded and mountainous 2,600-kilometre border region of Pakistan.

The U.S. operation is being labelled as a possible "end game of the war in Afghanistan," said CBC correspondent James Murray.

"There is a strong belief here that if the Taliban can be beaten and beaten back in Helmand province, they can be beaten almost entirely, if it's possible at all here in Afghanistan," Murray said.

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

"Where we go we will stay, and where we stay, we will hold, build and work toward transition of all security responsibilities to Afghan forces," U.S. Marine Corps Brig.-Gen. Larry Nicholson said in a statement.

Opium, heroin trade

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 operation aims to show "the Afghan people that when we come in, we are going to stay long enough to set up their own institutions," Capt. Bill Pelletier, a spokesman for the marines.

Once on the ground, the troops will meet with local leaders, hear their needs and act on them, Pelletier said.

With files from The Associated Press