The former US commander in Afghanistan, Stanley McChrystal, has urged that the draft be reinstated to spread the burden of fighting and to instill a sense of shared civic duty among young Americans.

The country's all-volunteer force has performed with great skill but after more than a decade of war "we're running very, very hard and at a certain point you can't expect it to go forever," McChrystal said at a conference last month.

Apart from the strain on troops and their families after repeated deployments to Iraq and Afghanistan, only a small fraction of the population was affected by the conflicts, the general said.

"It's less that one percent of Americans touched by this," he said at the event organized by the Aspen Institute.

Other senior military leaders have voiced concern about a growing gap between the all-volunteer force and the rest of society.

Former Pentagon chief Robert Gates said in 2010 that "for most Americans the war remains an abstraction" and Admiral Mike Mullen, the ex-chairman of the Joint Chiefs of Staff, said he feared US civilians "do not know us" while the military had become increasingly "insular."

But McChrystal's comments were unusual for advocating a return to some form of conscription. Most top brass are strong supporters of retaining a "professional" military and tend to oppose any talk of reinstating the draft, which was discarded in 1973 amid turmoil over the unpopular Vietnam War.

"We've never done an extended war with a professional army like this. We've got a very professional army, a volunteer army and professional reserve and they've done a lot. So we're in uncharted territory," said McChrystal.

The general, who was sacked in 2010 over a magazine profile in which he and his staff ridiculed top civilian leaders including the US president, said a new national service requirement could include a range of non-military activities.

"I'm becoming a little bit more extreme on this each year. I think we need national service and I think you need it either at the conclusion of high school or university.

"I don't think young people would really fight it if it was fair, if everybody did it. I'm not talking about military, I'm talking about all kinds of things," he said.

The specific projects were less important than the "shared experience" of service, he said.

"It's not whether they go build roads and parks or that sort of thing. It's what you put inside them, because once you have contributed to something, you have a slightly different view of it."