Tani, Afghanistan - Within clear sight of a Pakistani military observation post, Taliban insurgents launch a ferocious barrage of fire against two dozen US and Afghan army troops perched on a tiny frontier hilltop.

Caught off-guard by the mid-afternoon attack, the unit must weather a 20-minute onslaught with rocket-propelled grenades, machine-guns and automatic rifles until helicopter gunships come to its aid.

The attackers melt away and, according to the US troops, recross the border from Afghanistan to a sanctuary located about 3 kilometres inside Pakistan's restive tribal belt.

"It definitely all came from that training camp we've been telling people about," said Lieutenant Shane Oravsky of the 2/506 Infantry Regiment, 101st Airborne Division, whose platoon quickly recovers and digs in against a possible follow-up attack.

Amid a 30-per-cent surge in militant attacks in the border region this year compared to 2007, the lieutenant hopes an official complaint will be lodged with the authorities in Pakistan over the observation post's failure to respond to the incident.

But the clash is quickly overshadowed by broader tensions between Islamabad, Kabul and the international forces over the escalating conflict in the mountains. Following the hilltop attack, numerous reports cited US air strikes on targets in North Waziristan, a hotbed of militant activity that lies directly across the border.

While the US-led coalition forces said they had no information on the alleged strikes - a customary response in recent weeks - Pakistani security officials claimed at least half a dozen al-Qaeda fighters were among more than 20 people killed in the action.

The news could not sit well with Pakistan's new president, Asif Ali Zardari, widower of assassinated former Pakistani leader Benazir Bhutto.

In September, days after his inauguration and following an incident in which Pakistani troops fired on US helicopters buzzing around the border, Zardari declared "we cannot allow our territory and sovereignty to be violated by friends."

US Defence Secretary Robert Gates, however, maintains that the United Nations charter allowed the US to act in self-defence against international terrorists in Pakistan if the government was unable, or unwilling, to deal with them.

"We will do what is necessary to protect our troops," Gates said.

After seven years of often constrained post-9/11 cooperation between Washington and Islamabad, the war on terror in the region has reached a critical juncture.

While stressing the need to work closely with the Pakistani authorities, Gates recently expressed the opinion that after the successes exacted against al-Qaeda in Iraq, the greatest threat to the United States "lies in western Pakistan."

Successive attempts by Pakistan to subdue the region by force - invariably under intense pressure by Washington - resulted in painful losses.

No one is more adept at fighting in this inhospitable terrain than the militants, who while aided by foreign fighters from the Middle East, Chechnya and Uzbekistan, are largely drawn from the Pashtun tribes that inhabit both sides of the frontier.

A major improvement in the situation is that unlike authorities under former president Pervez Musharraf, the new Pakistani government does not flatly deny the presence of militant camps on its soil. Instead, it says it is determined to eliminate them.

But current efforts by Islamabad's forces to crush militant strongholds have only a limited scope. And although further US air strikes into the tribal belt may inflict a high cost on the enemy, they increasingly harm relations with Islamabad and fuel public anger in Pakistan at the armed intervention.

Meanwhile, ask US troops on the border what they think of their Pakistani allies and the common response bodes ill for cooperation prospects.

Some cite occasions when attacks were launched under the nose of Pakistani military units, which they say are either too outnumbered and afraid to fight or actively collude with the militants.

And for many of those in the forefront of the mountain war, the bottom line is: "We don't trust them." (dpa)