While speeches at the General Assembly were unrolling, behind the scenes diplomats were working on the text of a General Assembly resolution that they said they hoped to put to a vote later in the week. The nonbinding resolution would endorse Arab League efforts toward pursuing a cease-fire and a political solution in the conflict, they said.

In Washington, Turkey’s foreign minister, Ahmet Davutoglu, held lengthy talks on Syria with Secretary of State Hillary Rodham Clinton, exploring the possibility of additional diplomatic and economic steps. Both indicated that they would attend a meeting of nations in Tunisia on Feb. 24 to discuss support for Syria’s opposition after the Russia-China veto. Mrs. Clinton said the United States intended to increase humanitarian assistance to Syrians, but provided no details.

Mr. Davutoglu said that the focus remained on finding a political solution to the violence, which he added was possible “even at this moment,” but he also acknowledged that Turkish and other officials were considering additional steps that could involve safe havens or corridors that would require international troops.

“We cannot be silent when this humanitarian tragedy is continuing,” he said during remarks with Mrs. Clinton. “At this moment we are talking on diplomatic and humanitarian steps to be taken, but for other scenarios we hope that those things will not be needed, but we need to think about contingencies as well.”

Political analysts who have been following the conflict said there was little else the United States and its allies could do except strengthening economic sanctions against Mr. Assad, putting Syria’s major backers, notably Russia, on the defensive as the violence escalated, and offering assistance to Mr. Assad’s opponents.

“It’s the best shot we have,” said the Bruce W. Jentleson, a professor of public policy and political science at Duke University and a former senior adviser at the State Department. “The alternatives — to back off or go in militarily — are lousy options. It’s all the more reason to keep the pressure on.”