An Afghan man who emerged ablaze from a stolen pickup truck as an aircraft carrying U.S. Defense Secretary Leon Panetta landed at a base in Afghanistan has died from burns suffered in the incident, a U.S. commander said on Thursday.

The incident, an extraordinary security breach inside a military base in Afghanistan's south, coincided with the unannounced visit of the Pentagon chief on Wednesday.

It came three days after 16 villagers, most of them women and children, were killed in a shooting rampage blamed on a U.S. soldier that has raised questions about Western strategy in Afghanistan and heightened calls for an early withdrawal.

The shootings, and Wednesday's airfield incident, underscored the instability in Afghanistan more than 10 years into an increasingly unpopular war. They are the latest in a series of incidents that have added to anger among Afghans over the prolonged foreign presence.

The Afghan, a contractor who worked as a translator, had apparently tried to ram the truck into a group of U.S. Marines standing on a runway ramp at Camp Bastion in Helmand province, U.S. Lieutenant General Curtis Scaparrotti said.

Scaparrotti, second in command of U.S. forces in Afghanistan, told reporters travelling with Panetta he doubted the man had any idea Panetta was arriving at the heavily guarded base. Panetta and his delegation were unharmed.

I personally don't believe that it had any connection with the secretary's arrival, Scaparrotti said.

My personal opinion is yes, that he had an intent to harm, that he tried to hit the people on the ramp, he said.

Panetta is the highest-ranking U.S. official to visit Afghanistan since the shooting rampage in Kandahar province, which is next to Helmand and is the birthplace of the Taliban.

Although Panetta's trip was planned before the shooting, it comes as Afghan civilians and members of parliament demand answers.

Foremost among their demands is that the soldier responsible be tried in Afghanistan over the shooting, one of the worst of its kind since U.S.-backed Afghan forces toppled the Taliban in 2001 for harbouring the al Qaeda masterminds of the September 11 attacks on the United States.

Despite that, the U.S. staff sergeant accused of killing the 16 civilians had been flown out of Afghanistan, according to officials, as Washington scrambled to soothe anger over the massacre.

The New York Times, citing an unidentified senior U.S. official, said the soldier had been flown to Kuwait. CNN also reported the sergeant had been taken there.

Panetta told U.S. troops after he arrived the shooting must not deter them from their mission to secure Afghanistan ahead of a 2014 NATO deadline for the withdrawal of foreign combat troops.

Tension has risen sharply across Afghanistan since the attack and the inadvertent burning of copies of the Koran at the main NATO base last month, adding urgency to Panetta's visit. Panetta was to hold talks with Karzai and other Afghan leaders.

BOMBS, THREATS, PROTEST

The Taliban threatened to retaliate for Sunday's shooting by beheading U.S. personnel, while insurgents also attacked investigating Afghan officials on Tuesday.

U.S. soldiers are likely to be among those targeted in any reprisal attacks, although other Westerners have also been attacked after similar incidents and Afghan civilians invariably bear the brunt of upsurges in violence.

On Wednesday, at least nine people were killed in two separate bombings in Kandahar and Helmand provinces.

That followed demonstrations on Tuesday in an east Afghan city where protesters called on President Hamid Karzai to reject a strategic pact that would allow U.S. advisers and possibly special forces to remain beyond 2014.

In Washington, President Barack Obama said after meeting British Prime Minister David Cameron he did not anticipate any sudden change in plans for the pace of withdrawing troops.

Obama described the Kandahar massacre as tragic but emphasised at a briefing with Cameron that both nations remained committed to completing the Afghan mission responsibly.

In terms of pace, I don't anticipate at this stage that we're going to be making any sudden additional changes to the plan that we currently have, Obama said.

NATO leaders gathering in Obama's home city of Chicago on May 20-21 will decide the next phase of the planned transition to Afghan forces, which is already under way.

Scaparrotti offered a few more details about the airfield incident, saying it appeared the contractor had been carrying some kind of container that may have had fuel in it.

Those who were (there) described to me that (there was) a puff of smoke, and then the individual came out engulfed in flames. The security detachment there doused the flames and we took him for medical care, he said.

(Writing by Jack Kimball; Editing by Rob Taylor and Robert Birsel)