The Obama administration, which has firmly rejected calls to arm the Syrian opposition, appeared at least to allow for the possibility Tuesday by emphasizing that “additional measures” might have to be considered if President Bashar al-Assad continues to escalate his military assault on civilians.

The comments, made by spokespersons at both the White House and the State Department, came as dozens of international leaders prepared to attend an emergency meeting on Syria in Tunis on Friday.

Several governments in the region, including Qatar, the United Arab Emirates, Saudi Arabia and Turkey, have been at the forefront of efforts to take stronger action against Assad. The United States and others have been resistant.

The conference agenda includes discussion of humanitarian aid to Syrians under attack, additional sanctions against the Assad government, and efforts to help organize a divided internal opposition.

But shelling of Syrian opposition strongholds, including the city of Homs, by government forces has all but assured that the subject of arming the opposition will be on the table at the “Friends of Syria” gathering, where Secretary of State Hillary Rodham Clinton will head the U.S. delegation.

Russia, a Syrian ally whose veto of a U.N. Security Council resolution this month stymied a U.N. attempt to force Assad to step down, has said it will not attend, nor will Iran, Assad’s other main backer.

The Tunis meeting is patterned after a “Friends of Libya” conference that preceded international military intervention in that country. But administration officials said the Libyan situation had been far different, with a united political and military force opposed to leader Moammar Gaddafi.

The administration has been disappointed at the inability of the largely Sunni Syrian opposition to unify and to persuade minority Shiites, Christians and other groups to join against Assad. The leadership and organization of opposition military forces, primarily defectors who have formed the Free Syria Army, remain relatively opaque.

“Until we’re a lot clearer about who they are and what they are, I think it would be premature to talk about arming them,” Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told CNN on Sunday.

But the ongoing assault on Homs, surrounded by government artillery, increasingly echoes Gaddafi’s threat to obliterate the Libyan opposition stronghold of Benghazi, the spark that led to outside intervention there.

The administration and its allies have additional concerns as the Syrian situation worsens. Al-Qaeda forces from Iraq have already conducted some attacks in Syria, a senior administration official said, and there are fears their involvement will increase if the violence continues. There are also worries that Assad might unleash Syria’s considerable stores of chemical weapons.

Meanwhile, the International Committee of the Red Cross appealed Tuesday to Syrian officials to allow a daily two-hour truce so it can aid the worst-affected areas.

“In Homs . . . entire families have been stuck for days in their homes, unable to step outside to get bread, other food or water, or to obtain medical care,” ICRC President Jakob Kellenberger said in a statement.

“The current situation requires an immediate decision to implement a humanitarian pause in the fighting,” Kellenberger said.