Syrian demonstrators raise posters showing Syria's President Bashar Assad as they perform an oath to defend their country at Sabe Bahrat square in downtown Damascus, Syria, on Friday, Dec. 16, 2011, during a rally to support President Bashar Assad. Syrian security forces opened fire on anti-government protesters after Friday prayers at several locations around the country, while the army sent reinforcements into a southern area where military defectors recently launched deadly attacks on regime troops. ( AP Photo/ Bassem Tellawi)
(CBS) -- The Obama administration, under renewed pressure to help end the violence in Syria, is working with allies on a united, international "challenge" to the Assad regime, demanding that Syria stop the siege on the central city of Homs within days and allow immediate medical and humanitarian aid to flow directly to aid agencies trying to help the city's battered civilians.
Secretary of State Hillary Rodham Clinton and other leading Arab and European diplomats - a group calling itself the "Friends of Syria" - met Thursday in London, on the sidelines of a conference on security in Somalia, to work on plans to get humanitarian aid into Syria.
According to a senior U.S. official, the group will "challenge the Assad regime to respond within days."
With more than 70 countries sending representatives to Tunisia on Friday for a hastily-assembled Friends of Syria meeting, Secretary Clinton is working on the details of what the Tunis conference should accomplish.
According to diplomats from the U.S. and other countries with knowledge of the talks, the Tunis wish-list is shaping up roughly like this:
1. A demand that Assad order a ceasefire in the attack on Homs and other cities and allow access for humanitarian aid to flow. The aid would be channeled to relief agencies already in Syria, supervised by the United Nations.
2. Increased recognition of the Syrian National Council - a Turkey-based organization claiming to speak for 60 Syrian dissident groups - as the leading political alternative to the Assad regime. The nod to the SNC would fall short of formal recognition, but the U.S. is behind an Arab League effort to give the SNC more international credibility. (It's worth noting the Arab League's transition plan for Syria already calls for Assad to step aside peacefully.)
3. Renewed efforts to have Arab states enforce ongoing, but often ignored economic sanctions against the Assad regime.
Left conspicuously off the wish list is the question of what happens if Assad refuses to bend to the group's demands. In the wake of the Russian and Chinese vetoes of a U.N. Security Council resolution condemning the violence in Syria and calling for Assad to leave, the U.S. is hoping the Tunis conference will serves as a new, united, international voice of outrage.
But that's only a hope.
What about arming the rebels? The U.S. is encouraging back-channel, unofficial discussions of how to send arms to rebel forces, but is reluctant to openly endorse an arms plan because it's unclear where those arms would end up if Syria descends further into civil war.