Gates said sensitive intelligence which could endanger informants had been widely distributed down to junior level in the US army, in a loose policy which might now have to be reconsidered.

"We endeavour to push access to sensitive battlefield information down to where it is most useful – on the front lines – where as a practical matter there are fewer restrictions and controls than at rear headquarters," he said. "In the wake of this incident, it will be a real challenge to strike the right balance between security and providing our frontline troops the information they need."

Admiral Mike Mullen, who chairs the joint chiefs of staff, said: "Mr Assange can say whatever he likes about the greater good he thinks he and his source are doing, but the truth is they might already have on their hands the blood of some young soldier or that of an Afghan family."

The Afghan president, Hamid Karzai, called the disclosure of the names of Afghans who had co-operated with Nato and US forces "irresponsible and shocking". He said in Kabul: "Whether those individuals acted legitimately or illegitimately in providing information to the Nato forces, their lives will be in danger."

WikiLeaks withheld some 15, 000 intelligence reports to protect informants. But some of the posted texts contain details of Afghans who have dealt with the coalition.

Assange said today that they had tried to comply with a private White House request to redact the names of informants before publication. But the US authorities had refused to assist them.

He said in a statement: "Secretary Gates speaks about hypothetical blood, but the grounds of Iraq and Afghanistan are covered with real blood."

Thousands of children and adults had been killed and the US could have announced a broad inquiry into these killings, "but he decided to treat these issues with contempt''.

He said: "This behaviour is unacceptable. We will continue to expose abuses by this administration and others."

Meanwhile, both US and UK authorities remained silent about the disclosures in the 92,000 war log files that hundreds of civilians have been killed or wounded by coalition forces in unreported or previously under-reported incidents. The Ministry of Defence withdrew promises to make an official statement about US allegations that two units of British troops had caused exceptional loss of civilian life.

MoD sources said that at least 15 of the 21 alleged cases had now been confirmed, but they were unable to say what investigations had subsequently taken place, or when they would now make a statement.

A detachment of the Coldstream Guards was newly arrived in Kabul when innocent civilians were shot on four separate occasions in October-November 2007.

Several different companies of Royal Marine commands are alleged to have shot civilians who came "too close" to convoys or patrols on eight occasions in Helmand province during the six-month period ending in March 2008.

Sources said that the then Labour foreign secretary, David Miliband, was so concerned about civilian deaths that he helped push forward a UN resolution in 2008, setting up an UN system to monitor such casualties.

But it does not function effectively, according to the independent Human Rights Watch. The United Nations Assistance Mission in Afghanistan reported 828 civilian deaths in 2008, thanks to "pro-government forces", saying force protection incidents, "are of continuing concern", where innocent drivers, car passengers or motorcyclists, are shot by passing troops.

The US authorities are concentrating their firepower on leakers and their friends. Gates said the FBI had been called in to widen the criminal investigation into Private Bradley Manning, who is in military custody charged with leaking a classified video showing Apache pilots gunning down two Reuters cameramen in Baghdad who they believed might be insurgents.

Manning is being moved from a military jail in Kuwait to Qauntico, Maryland, and the FBI will now be able to investigate civilians such as Assange, for possible conspiracy offences. Assange's whereabouts were unknown today.