Establishing a no-fly zone to protect Syrian rebels would require hundreds of U.S. aircraft at a cost of upward of $1 billion per month and no assurance that it would change the momentum in the 2-year-old civil war, the chairman of the Joint Chiefs of Staff said Monday.

In a letter to two senators, Gen. Martin Dempsey outlined the risks, costs and benefits of more aggressive U.S. military action as the Obama administration weighs the next steps in helping the opposition battling the forces of President Bashar Assad. Democratic Sen. Carl Levin, chairman of the Armed Services Committee, and Sen. John McCain, R-Ariz., had pressed Dempsey for his personal assessment before moving ahead with his nomination to another two-year term.

Dempsey spelled out costs, ranging from millions to billions, for options ranging from training and armed vetted rebel groups, conducting limited strikes on Syria's air defenses and creating a no-fly zone or buffer zone.

The military leader said that while these steps would help the opposition and pressure Assad's government, "we have learned from the past 10 years; however, that it is not enough to simply alter the balance of military power without careful consideration of what is necessary in order to preserve a functioning state."

Dempsey's reference was to the more than a decade of war in Iraq and Afghanistan.

The Joint Chiefs chairman said creation of a no-fly zone would neutralize Syria's air defenses. It would require "hundreds of hundreds of ground and sea-based aircraft, intelligence and electronic warfare support, and enablers for refueling and communications. Estimated costs are $500 million initially, averaging as much as a billion dollars per month over the course of a year."

He said that while it would likely result in the "near total elimination" of Syria's ability to bomb opposition strongholds, the risks would be the loss of U.S. aircraft. That would mean recovery efforts for American personnel.

He added that such a step "may also fail to reduce the violence or shift the momentum because the regime relies overwhelmingly on surface fires _ mortars, artillery and missiles."