US Defence Secretary Leon Panetta's visit to Afghanistan was marred by a serious security breach 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 Mr Panetta.

However, the incident raised questions over security at the fortified base and added to a crisis atmosphere in the NATO-led war effort.

Mr 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, has been flown out of Afghanistan, Pentagon officials said, without saying where he was taken.

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

The transfer of the suspect could complicate already difficult talks with Kabul on a possible US troop presence after 2014.

President Hamid Karzai's government has so far refused to grant legal immunity to US troops - the same issue that scuppered a US strategic pact with Iraq.

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

Even as Mr 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 Mr Panetta's aircraft was landing at 11am (6.30am Irish time), 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, Mr 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," Mr Little said.

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

US officials with Mr Panetta were aware of the incident soon afterwards but waited ten hours to tell the media.

No change in US withdrawal plan: Obama

US President Barack Obama has said he expected no "sudden" changes to troop withdrawals in Afghanistan, as he and British Prime Minister David Cameron vowed not to "give up" the mission.

A string of recent incidents, including a massacre of Afghan civilians by a US soldier on Sunday, and deep public fatigue with the war, have prompted speculation that the pace of troop withdrawals could be speeded up.

However Mr Obama, confirming that NATO planned to transition to a support role in Afghanistan in 2013 before a full withdrawal in 2014, said his existing plan - to bring home an additional 23,000 troops this summer - still stood.

"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," Mr Obama said at a press conference in the White House Rose Garden with Mr Cameron.

Mr Obama promised a "robust" coalition presence in Afghanistan during this year's fighting season to stop the Taliban regaining momentum, but left open the option of looking at the size of the foreign troop footprint after that.

Both leaders insisted the painful sacrifices endured by both their militaries had been justified by the rout of Al-Qaeda, which had prevented terror plots against their countries.

Mr Obama also took issue with commentary that the US Afghan war plan was unravelling and that hopes of leaving a functioning Afghanistan able to take care of its own security were unravelling.

"If we maintain a steady, responsible transition process, which is what we've designed, then I am confident that we can put Afghans in a position where they can deal with their own security," said Mr Obama.

"We have made progress."