US defence chief acknowledges troubling events in Afghanistan

US defence chief Leon Panetta on Wednesday acknowledged that "deeply troubling" events had challenged the war effort in Afghanistan, visiting the country days after a soldier shot dead 16 civilians.

But he vowed that fallout from the burning of Korans at a US base, Sunday's massacre by a US soldier and Afghan soldiers killing coalition troops would not derail the mission to defeat Al qaeda and reverse a Taliban insurgency.

Panetta flew to Kabul after talks in southern Afghanistan just days after a soldier shot dead 16 villagers -- most of them women and children -- in Kandahar in the worst single such incident since the 2001 US-led invasion.

Panetta's spokesman described a meeting with provincial leaders in Helmand as "excellent" but his talks were overshadowed by a bomb attack in the province that killed eight civilians, a stark reminder of the cost of the conflict.

In a sign of the extent to which nerves are on edge, US Marines waiting to hear Panetta speak inside a hall were asked to leave their rifles outside.

American troops typically have their rifles in hand when the US defence secretary addresses them.

A defence official told reporters there was no heightened security threat but the US troops had been ordered to leave their weapons at the door to be "consistent", as the Afghans in the hall were unarmed.

Addressing US, NATO and Afghan troops at Camp Leatherneck, Panetta conceded that the burning of Korans on a US base last month, Sunday's killings and attacks on coalition troops by Afghan soldiers had posed difficult challenges.

"Each of these incidents is deeply troubling," he said.

"We will not allow individual incidents to undermine our resolve.

"We will be tested, we will be challenged by the enemy, by ourselves and by the hell of war itself."

But he added that "thanks to your efforts, our strategy is working".