The top US general has informed Congress of options for military intervention in Syria, but stressed that the decision of whether to go to war was one for civilian leaders.

In a non-classified letter made public Monday, chairman of the Joint Chiefs of Staff General Martin Dempsey set out five options -- from nonlethal intelligence and weapons training to a boots-on-the-ground plan to "assault and secure" the Syrian regime's chemical weapons.

Saying he was mindful of ongoing deliberations over whether to intervene militarily against President Bashar al-Assad's regime, Dempsey wrote that such a decision is "a political one that our nation entrusts to its civilian leaders."

But his letter -- which was addressed to Senate Armed Services Committee chairman Carl Levin -- also detailed risks, such as the empowering of extremists and retaliatory strikes by the regime.

It also carried an ominous warning for a nation weary of war after more than a decade of armed conflict in Iraq and Afghanistan.

"Once we take action, we should be prepared for what comes next," he wrote. "Deeper involvement is hard to avoid."

The United States is currently providing humanitarian assistance and non-lethal aid to rebel groups battling to oust Assad.

President Barack Obama's administration promised an expansion of military aid to Syria's rebel forces in June after accusing the regime of using chemical weapons, but such aid has yet to be disbursed.

Beyond training, Dempsey said the United States could conduct lethal stand-off strikes that would degrade the regime's air defenses as well as ground, missile and naval forces.

Such an option would require hundreds of aircraft and ships and, "depending on duration, the costs would be in the billions."

Another option, one backed by hawkish Senator John McCain, is the establishment of a no-fly zone to prevent the regime from using its aircraft to bomb rebel areas.

A deeper commitment would be establishing buffer zones or so-called humanitarian corridors to protect areas such as those along the borders with Turkey and Jordan, where hundreds of thousands of refugees have fled.

Such a move could reduce human suffering, but would require lethal force to defend the zones from regime attacks -- and would likely cost more than $1 billion a month.

The most aggressive plan would entail "thousands of special operations forces and other ground forces... to assault and secure critical sites" that contain Syria's chemical weapons.

Obama and lawmakers including McCain have consistently shied away from plans requiring US ground forces in Syria.

Instead of looking at the options in isolation, Dempsey advocated a regional approach that isolates the conflict, prevents further destabilization and weapons proliferation, and helps develop a moderate opposition.