KABUL: 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".

During talks with Helmand provincial leaders, Pentagon spokesman George Little said the defence chief told them that "the United States remains focused on the mission, and that recent events will not deter us from carrying it out".

Panetta's third visit to Afghanistan as defence secretary coincides with a growing debate in Washington about the course of the 10-year war, with some sceptics arguing for an accelerated drawdown of troops.

Under current plans, the United States aims to reduce its force of nearly 90,000 to about 68,000 by the end of September, with most of the remaining combat troops due to be pulled out by the end of 2014.

US officials have left the door open to a smaller force after 2014 but the recent setbacks have complicated negotiations with the Afghan government for a long-term security agreement.

One major sticking point is President Hamid Karzai government's refusal so far to grant legal immunity to American troops -- the same issue that scuppered a US strategic pact with Iraq.

Panetta was to meet Karzai on Thursday in a bid to further reassure him after Sunday's killings in Kandahar province, bordering Helmand.

Suspected Taliban insurgents raised tensions further on Tuesday by opening fire on an Afghan government delegation attending a memorial service for the murdered civilians, killing an Afghan soldier and wounding a policeman.

The eight civilians who died on Wednesday were killed when a minibus ran over a roadside bomb in Marja district -- the focus of a major US-led military offensive designed to clear out the Taliban in early 2010.

Sunday's shooting spree marked the latest in a string of damaging incidents that have raised questions about the NATO war effort, including the burning of Korans that triggered unrest in which some 40 people were killed.

Panetta portrayed the shooting as an isolated incident and insisted the US-led war effort was on track, with the Taliban in retreat.

Karzai described the massacre as "unforgivable" and the Afghan parliament wants the suspect tried locally -- but Panetta made clear the soldier would face US military justice, and possible execution.

But there are also growing concerns that Afghan soldiers, being trained to take over security when US and NATO allies pull out, are increasingly turning their weapons against Western troops.

Nearly one in five NATO soldiers killed this year has died at the hands of their supposed allies -- six of them Americans who were killed by Afghan colleagues after the Koran burning. AGENCIES