James M. Lindsay, the director of studies at the Council on Foreign Relations, said of the Security Council’s decision: “It’s going to be tougher to stop Qaddafi today than it was a week ago. The issue is not going to be settled in the skies above Benghazi, but by taking out tanks, artillery positions and multiple-launch rocket systems on the ground.”

Mr. Lindsay said that would require helicopter gunships and other close-in support aircraft rather than advanced fighter planes. Other analysts said repelling Colonel Qaddafi’s forces might require ground troops, an option that has been ruled out by senior American officials.

A Pentagon official said Thursday that decisions were still being made about what kind of military action, if any, the United States might take with the allies against Libya. The official said that contingency planning continued across a full range of operations, including a no-fly zone, but that it was unclear how much the United States would become involved beyond providing support.

That support is likely to consist of much of what the United States already has in the region — Awacs radar planes to help with air traffic control should there be airstrikes, other surveillance aircraft and about 400 Marines aboard two amphibious assault ships in the region, the Kearsarge and the Ponce.

The Americans could also provide signal-jamming aircraft in international airspace to muddle Libyan government communications with its military units.

A European diplomat said that Britain and France were still waiting to hear what role the United States would take in any military action in Libya. “One decision that needs to be made,” he said, “is whether there will be a command and control operations in Britain or in France.”