The US Army leaders warned Monday that budget cuts may force them to cut the size of force too deeply at a time of proliferating security challenges.

The warnings come as the army rolls out a new strategic vision that calls for reshaping the force to meet unpredictable demands in a fast-changing world situation.

General Raymond Odierno, the army chief of staff, said the advisability of plans to shrink the army from 490,000 to as low as 420,000 by the end of the decade is being thrown into question by events.

"The world is changing in front of us," he told reporters. "We've seen Russian aggression in eastern Europe, we've seen ISIS, we've seen some increased instability in other places."

"So I now have some concerns whether to go below 490 is the right thing to do or not, because of what I see potentially on the horizon," he said. "But we don't have the money to do that right now."

In the face of legally imposed government-wide budget cuts, the army has opted to cut troop strength to save money for weapons modernization and training and readiness programs -- but even those are at risk, Odierno and Army Secretary John McHugh said.

"We continue to increase the requirements of commitments of our forces and yet we cannot assure we are going to have the readiness to meet those commitments," Odierno said.

Both men said they saw little relief from sequestration until a new Congress settles in after next month's mid-term elections, but Odierno urged a debate on national security strategy that puts "all of it on the table."

Meanwhile, the army is moving ahead with a new "operating concept" that highlights the difficulty of planning for future conflicts in the current environment.

Called "Win in a Complex World," it is being touted as the biggest change in army forward thinking since the Cold War, when it was organized to fight land battles against the former Soviet Union with tanks, assault helicopters, and air defense missiles.

To the extent that the new concept is embraced by the service, it will shape the way the army equips and trains its forces over the next 20 years.

"We've got to begin the intellectual change right now," Odierno said.

The new concept stresses the creation of an adaptable force whose officers are encouraged to solve unforseen problems, rather than fight to set battle plans.

"The army cannot predict who it will fight, where it will fight, and with what coalition it will fight," General David Perkins, the commander of the army's Training and Doctrine Command said, introducing the document.

It envisions an army designed to operate closely with other military services, allied forces, civilian agencies and non-governmental organizations.

This marks a shift in emphasis from a heavy reliance on high-tech weaponry to an officer class trained and capable of thinking outside the military box.