Stressing the potentially dangerous effects on morale, the director of the Centre for Research on Military Organisation said "there is no question that the force is stretched too thin".
Speaking from the University of Maryland on Thursday, David Segal added: "We have stopped treating the reserves as a force in reserve. Our volunteer army is closer to being broken today than ever before in its 30-year history."
Defence Secretary Donald Rumsfeld and US commanders concede that the 1.4 million-member active duty armed forces, which have been cut by about a third since the end of the Cold War, are stretched.
Deployments in South Korea and Europe as well as post-2001 wars in Afghanistan and Iraq are a huge drain on resources.
But Rumsfeld says he has seen no evidence so far in a major ongoing Pentagon study to support calls from analysts and some Army officials to boost the service's strength by perhaps 20,000 troops to 500,000.
Signs of strain are appearing, however. Segal said the National Guard finished last year around 10,000 below its recruitment target and he predicted more severe recruitment and retention problems next year.
Rumsfeld says military stretched
but not over-streteched
To stem losses, the army has started offering re-enlistment bonuses of up to $10,000 to soldiers in Iraq, Afghanistan and Kuwait.
At the same time, it is preventing soldiers rotating home from retiring or leaving the service for up to 90 days after returning to their home bases.
The Army alone has blocked the departure of more than 40,000 soldiers, about 16,000 of them National Guard and reserve members who were eligible to leave the service this year, the Washington Post reported this week.
The Pentagon said that 187,746 National Guard and Reserve troops were mobilized as of 31 December 2003. About 20% of the troops in Iraq are reservists or Guard members but this proportion is expected to double next year.
Their enforced service has created major financial and emotional difficulties for many.
Last month, a group of angry reservists sent out an e-mail entitled "Chained in Iraq" complaining that their businesses and careers in the civilian world were being ruined by their enforced absence.
Karen, 28, an air traffic controller, was supposed to leave the Navy in December 2001 but her retirement was frozen after the September 11 attack on the World Trade Centre.
Eventually she signed on for two extra years after being promised she would be posted to the same base as her husband. Instead, she wound up in Iraq.
"We have stopped treating the reserves as a force in reserve. Our volunteer army is closer to being broken today than ever before in its 30-year history."
director, Centre for Research on Military Organisation
The two years have expired but she recently received notice of another eight-month deployment there starting next month. After five years of marriage, Karen, who did not want to be named or directly quoted, has never lived with her husband.
Loren Thompson of the Lexington Institute, a non-partisan think-tank, said: "The fact that the force is stretched so thin creates conditions that make more people anxious to leave. We're darned close to this becoming a serious operational issue."
Rumsfeld said this week the number of troops being prevented from leaving was relatively small and the military had been able to retain the numbers of people it needed.
"We've got a number of programmes underway to reduce stress on the force today. I think that there's been a very positive response to the way that this is being managed," he said.
Analysts believe some strains are inevitable as the force is remodelled to make it more flexible and based more on high tech weapons than boots on the ground.
General Richard Myers, chairman of the Joint Chiefs of Staff, added: "We're asking extraordinary things from the force and their families.
"I think most individuals understand and their families understand what we're asking them to do. We're asking them a lot. They're responding magnificently."