BERLIN (AP) -- The alleged slaying of 16 civilians by a U.S. soldier in southern Afghanistan will not affect plans to turn over security operations to local troops, NATO and member countries said Monday, calling for a quick investigation into the deaths.
NATO spokeswoman Oana Lungescu called for a swift and thorough investigation, but said it was an "isolated incident" and as such would not affect the alliance's overall plans to turn over security operations to Afghan troops by the end of 2014.
"This is clearly a tragedy, but it should not be allowed in any way to change the mission, and it doesn't change our commitment to the mission and to the timetable," she told The Associated Press in a telephone interview.
British Prime Minister David Cameron echoed that sentiment, calling the killings "absolutely dreadful" but insisting NATO strategy in Afghanistan must not be derailed. "We must stick to the plan and deliver the plan as we set it out," he said in London.
U.S. officials said the shooter, identified as an Army staff sergeant, acted alone after leaving his base in southern Afghanistan. Initial reports indicated he returned to the base after the shootings and turned himself in. He was in custody at a NATO base in Afghanistan.
Canadian Defense Minister Peter MacKay, whose country's troops are primarily located in Kandahar province where the shootings took place, said the attack was "deplorable and runs contrary to everything that the international mission to Afghanistan aims to accomplish."
But at the same time, he said it could not be allowed to affect the work between ISAF troops and their Afghan counterparts.
"That work will not be deterred by a random and cowardly act of violence," he said.
On a trip Monday to visit German troops in the northern city of Mazar-e-Sharif, German Chancellor Angela Merkel even suggested 2014 may be overly optimistic, saying there had been some progress in Afghan reconciliation efforts but not to the extent that Germany is able to say it could withdraw.
"So I cannot yet say, will we manage that by 2013-14," she said. "The will is there, we want to manage it, and it is being worked on."
Germany is one of the biggest contributors to the NATO-led International Security Assistance Force after the U.S. and Britain, with some 4,800 troops serving primarily in northern Afghanistan. Overall ISAF has some 130,000 troops.
The Taliban has vowed revenge for the killing of the sleeping villagers, though there were no immediate attacks or signs of protests on Monday. Following news that U.S. troops had burned Qurans last month, the Taliban claimed responsibility for several attacks and some Afghan forces also turned their guns on their allies, killing six U.S. troops as violent protests wracked the country.
Following Sunday's shooting, German Development Minister Dirk Niebel -- who was in Afghanistan for talks in Kabul with Afghan officials -- said the security situation was "tense" on Monday and condemned the attack.
"Mutual trust and understanding for the other's culture are indispensable for successful involvement in Afghanistan," Niebel said in a statement.
German Brig. Gen. Carsten Jacobson, a spokesman for the NATO-led International Security Assistance Force, said all forces had upped their security precautions out of concern for retaliatory attacks.
The former commander of British forces in Afghanistan, Col. Richard Kemp, said the shootings might cause an uptick in attacks, and "a short period of anger among local people in the south of the country." But the greater worry is that they could cause people to distrust NATO forces, which means they will be less likely to provide intelligence on the Taliban, he said.
"One of the most important things that our forces have achieved is winning over the confidence and trust of the local people so they support the government of Afghanistan and not the Taliban," he said. "That's one of the main objectives of our counter insurgency strategy. It has great operational importance."
Col. Ulrich Kirsch, the head of the Deutsche Bundeswehrverband -- which represents Germany's servicepeople -- said German soldiers would likely see the attack Sunday as an isolated incident, so it should not affect the "super" relations enjoyed with their American counterparts in Afghanistan.
But he said he suspected the reaction by the Afghan people would be more emotional, and could lead to increased violence.
"It certainly doesn't make the security situation any easier, but everyone who earnestly wants to succeed will not blame everyone for the actions of an individual," he said.
NATO countries immediately sought to mitigate the possible damage, condemning the attack and promising a full investigation.
Germany's Foreign Minister Guido Westerwelle said it was a "setback for the strong commitment of the international community for peace and reconciliation in Afghanistan" and called for calm.
"It would be yet another tragedy if more people were harmed in reaction to this horrible act," he said.
Associated Press Writers Cassandra Vinograd and Meera Selva in London and Charmaine Noronha in Toronto contributed to this report.