A group of prominent US defense and national security experts sounded an alarm Tuesday about the strain on US combat forces of lengthy deployments to Iraq, saying the problem has reached crisis levels.

The National Security Advisory Group, chaired by former Secretary of Defense William J. Perry, alleged in a letter to top congressional Democrats that the US administration's underfunding of the army represents "a serious failure of civilian stewardship of the military."

"Two-thirds of the army's operating force, active and reserve, is now reporting in as unready," the group wrote in their letter to lawmakers.

"There is not a single non-deployed Army Brigade Combat Team in the United States that is ready to deploy.

The letter continued: "The bottom line is that our army currently has no ready, strategic reserve. Not since the Vietnam era and its aftermath has the Army's readiness been so degraded."

Members of the group comprise a Who's Who of moderate-to-liberal political thought in the United States, including former Secretary of State Madeleine Albright, former national security adviser Samuel "Sandy" Berger, retired Joint Chiefs of Staff chairman John Shalikashvili, and retired four-star general and former presidential contender Wesley Clark.