By Rob Taylor -OUTPOST TERRANOVA - Afghanistan (Reuters) –

Private Brandon King was standing regular guard duty in a watch tower near Charqulba village in southern Afghanistan when he was killed by a single shot that was anything but normal.

King was hit in the head from long range, raising fears that foreign fighters with exceptional skills may have reinforced local Taliban as U.S. troops prepare an assault on insurgent strongholds, the area's U.S. battalion commander said.

"I would characterize them right now as out of the area," Lieutenant-Colonel David Flynn told Reuters. "What country they are from I couldn't necessarily tell you. But the skill of the enemy fighter that took down my soldier the other day is not something that was trained here," Flynn said.

King's unit was based just outside Charqulba at Combat Outpost Nolen, now experiencing daily attacks from insurgents operating out of the deserted village below. Local people have long since fled the fighting.

The Arghandab river valley is critical to the upcoming NATO and U.S. battle in Kandahar city to the southeast, as insurgents there have been able to slip into the city using its fertile grape and pomegranate fields for cover.

Southern Arghandab's farmlands are the district's last cover and concealment zone, in a maze of mud-walled villages known to U.S. commanders as "the triangle".

The governor here was recently assassinated and his replacement is now on what troops call the "rock star" tour, trying to win over local tribal elders in a series of high -- security village visits.

Flynn, who commands an artillery battalion recast as infantry and belonging to the U.S. 101st Airborne Division's 2nd "Strike" Brigade -- newly arrived in Kandahar -- said foreign fighters were coming from Pakistan and traveling west.

With local insurgents, they are also seeding the area with roadside bombs. Three soldiers lost limbs over the past week, while two soldiers were injured on Monday and airlifted out by helicopter for treatment.

A forensic investigation is underway into King's death, Flynn said, to determine whether the bullet was from a specialized sniper rifle or just a freak AK-47 shot.

But King's fellow soldiers, who will have a memorial service on Tuesday, have few doubts.

His base, COP Nolen, is now notorious across the battalion and troops believe the bullet was fired from more than 400 meters (yards), sapping morale at all combat posts in the area.

Most insurgents in the valley are generally unskilled and when in contact with U.S. troops, fire wildly with AK-47s or rocket propelled grenades (RPGs), Flynn said. Most RPGs fired were causing only concussion injuries.

"We have fortunately not yet until now seen in this area very skilled markmen," he said. "I don't think the Taliban has a marksmanship program."

While the pool of potential fighters was virtually without limit, Flynn said there were probably only 40-50 hardcore insurgents in his eight kilometer (five mile) section of the valley, home to 57,000 people.

Over coming weeks, U.S. and Afghan army troops will launch a joint offensive to choke out the insurgents and eventually re-forge government ties in the area.

Afghan troops began patrolling with U.S. soldiers this week and are learning advanced battle skills while living side-by-side with American troops on scorching tented bases.

U.S. soldiers believe they will be most vulnerable in the first nine days of operations, and in that time Afghan soldiers will be crucial to keeping them alive, using their local understanding of culture and possible threats.

"It's a field-to-field fight, so unless I put a combat outpost in every field, it's an endless game," says Flynn. "The answer has got to come from the people."

(Editing by David Fox and Sanjeev Miglani)