Establishing a no-fly zone to protect Syrian rebels would require hundreds of U.S. aircraft at a cost of more than $1 billion per month, with no assurance that it would change the momentum in the civil war there, the chairman of the Joint Chiefs of Staff said on 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 al-Assad.

Senator Carl Levin, chairman of the Armed Services Committee, and Senator John McCain had pressed General Dempsey for his personal assessment before moving ahead with his nomination to another two-year term.

General Dempsey spelled out costs, ranging from millions to billions of dollars, for options ranging from training and arming vetted rebel groups, conducting limited strikes on Syria’s air defences and creating a no-fly zone or buffer zone.

The military leader said that while these steps would help the opposition and pressure Mr. 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”.

Syrian rebels warn civilians

Activists say an al-Qaeda-linked group has warned civilians not to use a road linking central Syria with the northern province of Aleppo, declaring it a military zone.

The Britain-based Syrian Observatory for Human Rights and the Aleppo Media Centre said Tuesday that Jabhat al-Nusra, or the Nusra Front, is threatening to target any vehicle using the road starting Wednesday. A copy of the warning was posted online.

Activists have reported heavy fighting near the road, which links the central province of Hama with the embattled city of Aleppo.

The regime uses the route to ferry supplies to its forces in the north because the rebels already have severed the main north-south highway that connects Damascus with Aleppo.

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