WASHINGTON (AP) -- Establishing a no-fly zone to protect Syrian rebels would require hundreds of U.S. aircraft at a cost as much as $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 cautionary assessment of more aggressive American military action.
In a letter, Gen. Martin Dempsey outlined the risks, costs and benefits of five potential steps as the Obama administration weighs its next move to help the opposition battling the forces of President Bashar Assad. The sectarian conflict has killed an estimated 93,000 and displaced millions, prompting more calls on Capitol Hill for greater American action.
Dempsey said the decision to use force in Syria is not one to be taken lightly.
"It is no less than an act of war," he wrote. And once that decision is made, the U.S. has to be prepared for what may come next. "Deeper involvement is hard to avoid," he said.
The United States has been providing humanitarian assistance to the opposition seeking to overthrow the Assad government. The administration has recently taken steps to arm rebels with weapons and ammunition, a step welcomed by some in Congress but troubling to other lawmakers.
Separately, members of the House intelligence committee who had balked weeks ago at the Obama administration's first attempt to pay for lethal aid for the Syrian rebels said Monday that their concerns had largely been addressed.
"After much discussion and review, we got a consensus that we could move forward with what the administration's plans and intentions are in Syria consistent with committee reservations," Rep. Mike Rogers, R-Mich., chairman of the House panel, said in a statement.
At Dempsey's confirmation hearing last week for another two-year term, Armed Services Committee Chairman Carl Levin, D-Mich., had asked the general for his unclassified view of options for using U.S. military forces in Syria. Separately, Levin and Sen. John McCain, R-Ariz., a member of the committee, had pressed Dempsey about possible actions in Syria and risks associated with Afghanistan.
Tara Andringa, a spokeswoman for Levin, said Monday that the senators are expecting a separate response from Dempsey to their letter.
Responding to Levin, 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, creating a no-fly zone, establishing a buffer zone and controlling Syria's massive stockpile of chemical weapons.
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 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."
Dempsey said creation of a buffer zone, most likely a geographic area across the border with Turkey or Jordan, would give opposition forces a place to organize and train. Such a move would require thousands of U.S. ground forces, even stationed outside Syria, to back up those defending the zones.
That combined with U.S. ground forces would prove costly, at more than $1 billion per month, he said.
"We must also understand risk-not just to our forces, but to our other global responsibilities. This is especially critical as we lose readiness due to budget cuts and fiscal uncertainty. Some options may not be feasible in time or cost without compromising our security elsewhere," Dempsey wrote.
Last week, McCain and Dempsey tangled at the Army general's confirmation hearing. The GOP lawmaker said he would block Dempsey's nomination until he got an adequate response from the senior military official.