Efforts to prevent the massacre of 16 civilians by a US soldier in Afghanistan from derailing the international military strategy were stepped up on Monday as western political leaders tried to portray the incident as an isolated case.

The killings on Saturday were "inexplicable," said the US secretary of state, Hillary Clinton, who insisted that the episode would not force a change of course.

"This terrible incident does not change our steadfast dedication to protecting the Afghan people and to doing everything we can to build a strong and stable Afghanistan," she said as a White House spokesman insisted that the US and other Nato members remained on course to hand over security control to Afghans by the end of 2014.

"We recognize that an incident like this is inexplicable and will certainly cause many questions to be asked," Clinton told reporters at the United Nations in New York.

"But I hope that everyone understands in Afghanistan and around the world that the United States is committed to seeing Afghanistan continue its move toward a stable, secure, prosperous, democratic state."

A similar line came from the Pentagon, where a spokesman described the killings as a deplorable but "isolated incident" and said that the indications were that they were perpetrated by a single individual "acting on his own."

The White House released a picture of President Obama speaking on the phone with the Afghan president, Hamid Karzai, who could now harden his stance in talks with the US over an agreement that will be the framework for US involvement in Afghanistan after foreign combat troops leave at the end of 2014.

David Cameron, who is due to fly to Washington on Tuesday for talks with Obama, described the killings as "an absolutely dreadful event" but insisted: "We must stick to the plan and deliver the plan as we set it out."

"This really is an absolutely appalling thing that has taken place and, of course, it will have its impact, but we must do everything we can to make sure it doesn't in any way derail the very good work that American and British and other Isaf forces are doing in Afghanistan."

The prime minister acknowledged that it was "very difficult" but said it was essential to continue the transition to Afghan forces ahead of the withdrawal of most international troops by the end of 2014.

He was backed by the former head of the British army, General Lord Dannatt, who acknowledged the killings had been a setback to the mission, but said that a "precipitate" withdrawal of international forces would be a mistake.

"If we were to pull the plug and scuttle and run now we would not be able to complete that process of handing over to the Afghans and we would undoubtedly prejudice all the good work we have done over the years at considerable cost in both blood and treasure," he told BBC News.

The Canadian defence 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."

He said it could not be allowed to affect the work between Isaf troops and their Afghan counterparts, adding: "That work will not be deterred by a random and cowardly act of violence".

The German development minister, Dirk Niebel, who was in Kabul for talks with Afghan officials, said the security situation was "tense" on Monday.

"Mutual trust and understanding for the other's culture are indispensable for successful involvement in Afghanistan," Niebel said in a statement.

The German chancellor, Angela Merkel, who was on Monday visiting German troops in the northern city of Mazar-e-Sharif, said 2014 may be an overly optimistic date for withdrawal.

She said 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."

Other voices warned that time was running out for the Western allies' strategy in Afghanistan. The Labour MP David Winnick told the House of Commons that public support for what was increasingly seen as an "unwinnable war" was falling away.

"It won't help the fight against terrorism, perhaps it even helps the terrorists," he said.

The shadow foreign secretary, Douglas Alexander, told BBC Radio 4's Today programme that the narrowing window for securing a political settlement should be top of the agenda when Cameron meets Obama.