US Defence Secretary Leon Panetta's visit to Afghanistan was marred by a serious security breach Wednesday when a stolen vehicle was driven onto a runway ramp as the Pentagon chief's plane was landing at a NATO base.

US officials insisted there was no sign the incident at Camp Bastion in the country's south was an attempted attack on Panetta but the carjacking raised questions over security at the fortified base and added to a crisis atmosphere in the NATO-led war effort.

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

The suspect in the massacre, a US Army sergeant who had served three tours in Iraq, had been flown out of Afghanistan, Pentagon officials said Wednesday, without saying where he was taken.

The move indicated the suspect would, if charged, be tried in a US military court outside of the war-torn country, despite demands by Afghan political leaders for a public trial in Afghanistan.

The transfer of the shooting suspect could complicate already difficult talks with Kabul on a possible US troop presence after 2014, as President Hamid Karzai's government has so far refused to grant legal immunity to American troops -- the same issue that scuppered a US strategic pact with Iraq.

In the first leg of a two-day visit to Afghanistan, Panetta said recent "troubling" events should not force a change in NATO's war strategy.

Even as Panetta touted progress on the battlefield, officials reported a hijacked vehicle had made it onto a runway ramp where the Pentagon chief's plane was due to park at Camp Bastion in Helmand province.

At about the same time Panetta's aircraft was landing at 11:00 am (0630 GMT), an Afghan civilian hijacked a pick-up truck from a soldier in the coalition force.

He drove the vehicle at high speed before he crashed into a ditch and emerged in flames, Panetta's spokesman George Little told reporters in Kabul.

"Security personnel responded and for reasons that are totally unknown to us at this time, our personnel discovered he was ablaze," Little said.

The flames were extinguished and the Afghan was being treated for serious burn injuries, he said.

Despite Sunday's shooting spree and a series of violent incidents, including unrest over the burning of Korans at a US base last month, President Barack Obama said there were no plans for "sudden" changes to a scheduled timetable for troop withdrawal.

Obama said the United States would stick with the timing agreed with NATO partners, in which Afghan forces take over security for the whole country by the end of 2014.

"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 told a joint press conference with British Prime Minister David Cameron.

Details of the incident at the Camp Bastion airfield remain unclear.

Officials initially said a member of the NATO-led force was struck and injured by the truck. But a military spokesman in Washington, Captain John Kirby, later said the Afghan wounded the soldier when he hijacked the vehicle. The pick-up truck was driven at high speed "across the ramp near where (Panetta) was to pull up" at the airfield, Kirby told reporters.

No explosives or weapons were found on the Afghan or in the vehicle, he said.

"There is no evidence right now that the driver had any idea who was on that aircraft," Kirby said.

Reporters travelling with Panetta witnessed nothing out of the ordinary during the landing at the base. The secretary went ahead with his scheduled meetings with local Afghan leaders and addressed NATO and Afghan troops at Camp Leatherneck, which adjoins Britain's Camp Bastion.

Panetta then flew to Kabul where he is due to meet Karzai on Thursday.

US officials with Panetta were aware of the incident soon afterwards but waited 10 hours to tell reporters.