Chairman of the Joint Chiefs of Staff Army Gen. Martin Dempsey laid out five options the US military could undertake regarding Syria, in a letter to Congress released Monday.
Dempsey said intervention by the United States would likely strengthen the rebels, but even limited engagement could backfire.
He outlined the actions the military could take if ordered by the White House: training and advising rebels, conducting airstrikes, establishing a no-fly zone, establishing a buffer zone, and safeguarding the stockpiles of chemical weapons.
Dempsey cautioned that using any of those options would be "no less than an act of war."
"Once we take action, we should be prepared for what comes next," Dempsey said. "Deeper involvement is hard to avoid."
Establishing a no-fly zone over Syria would be a costly and complex affair, costing $500 million initially, but eventually averaging as much as a billion dollars per month, Dempsey said. The Syrian regime's use of artillery fire and mortars could pose a risk to American jets.
Having the military train rebels would cost another $500 million a year, and come with its own set of risks.
Dempsey's letter comes as the US House plans to debate a $598.3 billion defense spending bill Tuesday, with both tea party conservatives and liberal Democrats seeking to bar the Obama administration from arming Syrian rebels without congressional approval.
Specifically, the House will consider an amendment that would withold funds for intervention in Syria if the actions violated the War Powers Resolution.
CIA shipments of arms could head towards Syria as soon as the next few weeks, after the transfer of weapons was approved by two US congressional panels.
Despite reservations about where the arms could end up — for example, in the hands of Islamist militants — the House and Senate intelligence commmittees have given reluctant support to the administration's plan to arm Syrian rebels.