U.S. Defense Secretary Leon Panetta on Wednesday told troops in Afghanistan that the massacre of 16 Afghan civilians by an American soldier should not deter them from their mission to secure the country ahead of a 2014 NATO withdrawal deadline.

Panetta arrived in Afghanistan amid heightened tensions over the killings, underscored by a motorcycle bomb blast in Kandahar city that killed an Afghan intelligence soldier and wounded two others, as well as a civilian.

Some Afghan media said a vehicle burst into flames on the runway before Panetta's plane landed. NATO coalition forces played the incident down, saying that a vehicle had been stolen at Camp Bastion in Helmand, but gave few other details.

This incident took place this afternoon around the same time U.S. Secretary of Defense Leon Panetta was arriving in Afghanistan. At no point was the secretary or anyone on the aircraft in any danger from this incident, ISAF said. A roadside bomb also killed eight civilians in southern Helmand province, where Panetta began a two-day visit by meeting with U.S. and coalition forces at two bases, Afghan provincial officials said.

We'll be challenged by our enemy. We'll be challenged by ourselves. We'll be challenged by the hell of war itself. But none of that, none of that, must ever deter us from the mission that we must achieve, Panetta told soldiers at Camp Leatherneck, the main U.S. Marine base in the volatile area. As tragic as these acts of violence have been, they do not define the relationship between the coalition and Afghan forces and the Afghan people, he said. Panetta's trip had been scheduled before Sunday's shootings in two Kandahar villages, but gained added urgency as political pressure mounted on Afghan and U.S. officials over the unpopular war, now in its 11th year. Forty percent of Americans said the shooting spree, in which nine children and three women were among those killed, had weakened their support for the war, according to an online poll by Reuters/Ipsos.

MAJORITY WANT TROOPS HOME Sixty-one percent of Americans surveyed in the March 12-13 poll said remaining U.S. troops should be brought home immediately, down slightly from the 66 percent with that opinion in a similar March poll. Seventeen percent disagreed.

American soldiers are the likely targets of any backlash over the killings. The Afghan Taliban threatened to retaliate by beheading U.S. personnel, while insurgents also attacked investigating Afghan officials on Tuesday. But Panetta, the most senior U.S. official to visit Afghanistan since the shootings, said the massacre would not alter U.S. withdrawal plans and strategy. We will not allow individual incidents to undermine our resolve to that mission and to sticking to the strategy that we've put in place, Panetta told troops. Afghans investigating the incident had been shown video of the soldier, said to be a U.S. Army staff sergeant, taken from a security camera mounted on a blimp above his base, an Afghan security official who could not be identified told Reuters. The footage showed the uniformed soldier with his weapon covered by a cloth, approaching the gates of the Belandai special forces base and throwing his arms up in surrender, the official said. The video had been shown to investigators to help dispel a widely held belief among Afghans, including many members of parliament, that more than one soldier must have been involved because of the high death toll, the official said. Panetta was to hold talks with Afghan leaders including President Hamid Karzai as tension remains high following a spate of incidents including the burning of Korans at the main NATO base in the country last month. Panetta's arrival in Helmand - where U.S. Marines and British soldiers are battling a resilient insurgency - came a day after the first protests over Sunday's massacre flared in the eastern city of Jalalabad. Some 2,000 demonstrators chanted Death to America and demanded Karzai reject a planned strategic pact that would allow U.S. advisers and possibly special forces to remain beyond the pullout of most NATO combat troops by the end of 2014. DEMAND FOR TRIAL in AFGHANISTAN The U.S. military hopes to withdraw about 23,000 soldiers from Afghanistan by the end of the coming summer fighting season, leaving about 68,000. In the two Panjwai district villages where the weekend massacre took place, U.S. troops remained confined to the compound where the soldier was based, and people in the area demanded a trial in Afghanistan under Afghan law. They have to be prosecuted here. They have done two crimes against my family. One, they killed them, and secondly, they burned them, said Wazir Mohammad, 40, who lost 11 members of his family in the incident. A cleric, Neda Mohammad Akhond, said he believed the shootings may have been retaliation for an insurgent landmine attack on a U.S. convoy in the days before the massacre. They asked people to come out of their homes and warned them they would avenge this, Akhond said. There was no independent verification of an earlier attack. NATO officials said it was too early to tell if the U.S. soldier would be tried in the United States or Afghanistan if investigators were to find enough evidence to charge him, but he would be tried under U.S. laws. Typically, once an initial investigation is completed, prosecutors decide if they have enough evidence to file charges and then could move to a so-called Article 32 procedure or court martial. While Afghan members of parliament called for a trial under Afghan law, Karzai's office was understood to accept that a trial in a U.S. court would be acceptable provided the process was transparent and open to media.

(Additional reporting by Ahmad Nadem in KANDAHAR and Mirwais Harooni in KABUL, Writing by Rob Taylor; Editing by Kevin Liffey)