WASHINGTON -- The United States and Russia have agreed on a plan to pause the fighting in Syria's civil war at the stroke of midnight on Saturday, February 27, marking the first serious effort to broker a cease-fire in the conflict since it first began five years ago.The plan requires the two main warring parties – Syria's embattled president, Bashar Assad, and a loose alliance of rebel groups fighting for his ouster – to agree to cease fire by noon on Friday, local time. The truce would then go into effect at midnight.According to a joint US-Russian statement, the two sides must agree to fully implement a UN Security Council resolution passed in December, which lays out a political roadmap for an end to the war; To cease attacks with "any weapons, including rockets, mortars, and anti-tank guided missiles" against one another; To allow access for humanitarian aid to "people in need" nationwide; And to the use of "proportionate" force if the pause in fighting periodically falters.cessation of hostilities," which will only protect "those parties to the Syrian conflict that have indicated their commitment to and acceptance of its terms."That may not include some rebel groups which choose to continue fighting Assad, alongside if not in tandem with al Nusra, which occupies space in the same vicinity of several armed opposition groups supported by the US. The head of one Free Syrian Army group, Bashar al-Zoubi of the Yarmouk Army, expressed pessimism over this provision of the agreement, which he characterized as a potentially fatal flaw. "Russia and the regime will target the areas of the revolutionaries on the pretext of the Nusra Front's presence, and you know how mixed those areas are, and if this happens, the truce will collapse," he said.But the Obama administration responded to that criticism swiftly on Monday afternoon, defending their decision to exclude the terrorist organization from any cease-fire, as well as Washington's ability to identify its whereabouts."If you hang out with the wrong folks," State Department spokesman Mark Toner said, "you choose who you hang out with, and that sends a signal."A task force has been set up by the US and Russia– two governments on opposite sides of the war, currently co-chairing a working group of twenty nations seeking an end to the conflict– that will include a hotline to address challenges to the cease-fire and, if necessary, a working group to litigate disputes."We are all aware of the significant challenges ahead," US Secretary of State John Kerry said in a written statement. "Over the coming days, we will be working to secure commitments from key parties that they will abide by the terms of this cessation of hostilities and further develop modalities for monitoring and enforcement." "This is a moment of promise, but the fulfillment of that promise depends on actions," Kerry continued. "All parties must meet their commitments under this agreement, ensure full implementation of UN Security Council Resolution 2254, and cease attacks on each other, including aerial bombardments."UN Secretary General Ban Ki-moon praised the effort, which his spokesman characterized as "lengthy and detailed.""The agreement, if respected, would constitute a significant step forward in the implementation of Security Council resolution 2254," his press officer said. "It further contributes to creating an environment conducive for the resumption of political negotiations." That resolution calls for a cease-fire in the war, followed by negotiations toward a political transition, a national unity government and, ultimately, nationwide elections. None of these steps include negotiation with Islamic State, considered a terrorist organization by all parties to the larger civil war."Proximity" talks between Assad regime and opposition representatives– which feature diplomats from both sides in the same hotel, if not in the same room, engaging in dialogue through UN intermediaries– failed within just three days at the beginning of February, when rebels accused Assad and his allies in Moscow and Tehran of blocking crucial humanitarian aid to besieged areas. Russia has long cast all rebel groups– including those backed by the US, Turkey and the Gulf states– as terrorist organizations, and has increased attacks on key strategic areas in the state's northwest in recent weeks.According to their joint statement, the US and Russia agreed to "develop procedures necessary for preventing parties participating in the cessation of hostilities from being attacked by Russian Armed Forces," as well as the US-led coalition against Islamic State.US President Barack Obama and Russian President Vladimir Putin spoke by phone on Monday to seal the agreement, which Obama hailed, according to his press secretary, as an opportunity to "alleviate the suffering of the Syrian people, galvanize UN-led political process, and focus on defeating ISIL.""President Obama emphasized that the priority now was to ensure positive responses by the Syrian regime and armed opposition," White House press secretary Josh Earnest said.Putin hailed the agreement in a formal televised address, posted on the Kremlin's website."I'm convinced that the joint actions agreed with the American side are able to radically transform the crisis situation in Syria," Putin said. "The main thing is that conditions have been created for launching a genuine political process."The UN's special envoy to the Syrian crisis, Staffan de Mistura, had hoped to resume the talks in Geneva on Thursday. The new hope is for talks to resume in March, according to State Department officials, motivated by the potential success of a cease-fire and the fears of its failure."We can now relaunch very soon the political process which is needed to end this conflict," de Mistura said on Monday, in an e-mail to Reuters. "The Syrian people have been asking for two concrete signals after five years of endless war from the international community: stop the violence and, 'Please give us access to food and medicines.'""Both requests have finally started to be implemented," he added.Parties to the conflict also must agree to refrain from seeking to acquire any new territory at the expense of one another.UN-designated terrorist organizations– such as al-Qaida's affiliate in Syria, the al Nusra Front, and Islamic State – are excluded from the "