NATO was forced onto the defensive Monday over a humiliating attack on one of its most heavily guarded bases in Afghanistan that destroyed six US fighter jets in unprecedented damage in the 10-year war.

At a weekly press conference given by the US-led International Security Assistance Force (ISAF) at its closely guarded headquarters, chief spokesman Brigadier General Gunter Katz was pressed on Friday night's assault.

ISAF says it is still investigating how Taliban commandos, armed with suicide vests, guns and rockets and wearing US uniforms, breached the perimeter wall of Camp Bastion, in the southern province of Helmand.

The vast base -- where Prince Harry is deployed -- was deliberately built in the middle of the desert to have a vantage point.

The cost of the damage runs to tens of millions of dollars.

"Yes, we assess that this attack was well-organised, well-equipped and destroyed six Harrier jets, they damaged two additional Harrier jets and they destroyed buildings, they killed two US Marines, but we must not forget out of those 15 attackers, we killed 14 and captured one," Katz told AFP.

The Taliban said the assault was conducted to avenge an American-made film that insults Islam and which has sparked a violent backlash in Muslim countries across the world.

They also said that had they found Harry they would have killed him.

NATO insists the insurgency, now in its 11th year, is on the back foot with Afghan forces taking the lead over 75 percent of the population, as part of a phased departure of most Western troops by the end of 2014.

"Since the insurgency is clearly losing the fight and the security situation is becoming better everyday here in Afghanistan, they look for attacks that attract the media," Katz told reporters.

But he conceded that ISAF would learn lessons from the attack on Camp Bastion and adopt force protection measures accordingly, but declined to go into detail.

The raid kicked off a devastating weekend for NATO in which two British and four American soldiers were shot dead by suspected members of the Afghan police, and its warplanes were accused of killing eight women in an air strike.

Such "insider" attacks carried out by colleagues in the Afghan forces threaten NATO plans to hand over security to locals.

Concern is growing on how to halt them with 51 Western soldiers already killed in 36 such incidents so far this year alone.

In Japan, US Defense Secretary Leon Panetta sought to downplay such fears, calling the attacks a "last-gasp" tactic from Taliban who have lost ground in the last two years since a surge of NATO troops, now being withdrawn.

The US military is looking at further steps to protect troops, Panetta said, but insisted the attacks would not force a change in war strategy.

Analysts said the Bastion attack showed how well trained insurgents had become.

"It showed the Taliban no longer attack blind. They have learnt to plan attacks and train their fighters for them. After a decade the Taliban have also learnt from and know their enemy, they have become hi-tech," said Afghan analyst Waheed Mujda.

Others speculated about possible help from turncoats within the base, home to around 28,000 personnel.

"It would not have been possible without any help from inside. I'm sure they've had people among the Afghan police and army on the base," said Kabul-based analyst and writer Ahmad Saeedi.

NATO is gradually withdrawing its 112,600 remaining troops. The Pentagon said last week that there are currently 77,000 US troops in Afghanistan.