Khost, Afghanistan - Less than six hours after a helicopter drops a combined US and Afghan army patrol by the mountain border with Pakistan, Islamic insurgents surround and pound the unit into disarray.

The surprise attack from forested slopes around the troops' positions lasts 20 minutes, during which they are pinned down and barely able to hit back, such is the intensity of the barrage of fire. Then abrupt silence replaces the crash and rattle of rocket-propelled grenades and small arms.

"They know to pull back before air support can arrive," said Sergeant Ryan Hendricks of the 2-506 Infantry Regiment, 101st Airborne Division, as he positions the two dozen soldiers against any attempt to directly storm the hilltop they are holding.

True to form, the militants slip away before Apache helicopter gunships reach the site, ready to strafe the enemy with miniguns and rockets. The pilots find no trace of the attackers, who are thought to have hidden in nearby houses before escaping back across the frontier to a camp into Pakistan's tribal belt.

Amid rising tensions between US and Pakistani authorities over how to prosecute the campaign against Taliban and al-Qaeda marauders, the soldiers on the ground are up against numerous obstacles as they carry on the fight as best they can.

There are some 32,000 US troops in Afghanistan, but lack of manpower tells in every operation in these huge tracts of highland wilderness, where the enemy, drawn largely from the Pashtun tribes that span the porous border, is completely at home.

"I preferred it in Iraq," said Sergeant Bruce Hunter, a member of another platoon deployed in the area. "It was an urban fight, more fast paced, more chances to kill the enemy - here they shoot at you from a ridge and you'll never catch up with them."

Meanwhile, strict rules of engagement - partly a product of protests by the Afghan government in Kabul at civilian deaths during coalition operations - mean that threat verification must be absolute before the troops can engage.

Playing on the restrictions, the militants will avoid carrying their weapons where possible, hiding them in caches in the rocks until they are needed and enabling them to pose as innocent farmers if sighted.

Unless they are caught red-handed with arms it is hard to take action, no matter how suspicious their behaviour. "It's like trying to catch a criminal, you need a number of reliable sources and evidence," said Captain Ricardo Bravado, whose men were attacked on the hilltop.

Armaments may be brought to Afghanistan on mule trains from sanctuaries in Pakistan, moved along remote mountain trails or concealed beneath huge piles of firewood on commercial trucks.

Not even with assistance of the Afghan police and army is it practical to search the throng of vehicles that ferry goods between the countries. And curbs on access of US troops to private homes raises their dependency on Afghan government forces during operations.

Seven years after ousting the Taliban from power, the US military is also waging a separate campaign from the "hot war" in the eastern regions. The aim is to close the gap with local Pashtun tribes that are tired of violence and economic stagnation but remain wary of the foreign and Afghan government forces.

On a recent morning, airborne troops in the restive Khost province staged a meeting at an army base with some 80 mullahs and tribal elders and Afghan National Army officers to help establish a dialogue between the sides.

"This is a demonstration of good faith, that we can sit in a room together and be civil," said Captain Eric Robinson, who organized the event comprising a series of speeches by key figures about challenges and prospects of cooperation and a sit-down lunch.

With many tribes still sitting on the fence, the occasion was not expected to work miracles. But it was at least hoped to provide a forum for voicing and addressing suspicions and grievances, and perhaps help nudge some participants away from the Taliban's orbit in the long-term.

"There are people in the room who are strong sympathizers with the insurgents but that's OK - we are here to talk to them too and show we are not monsters and do listen," Robinson added. (dpa)