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Editor’s Note: Tim Lister is a CNN journalist who has lived and worked in the Middle East, and has reported on the region for more than two decades.

(CNN) —  

It’s day five of the “reduction of violence” in Syria, and even if the guns haven’t fallen entirely silent the death toll among civilians has shown a marked decline. But accusations of bad faith continue to fly in all directions, and the next steps to consolidate the relative quiet are an even greater challenge.

What the deal envisages

Many of the details hammered out between US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov a week ago have not been disclosed. But the essentials are these: A period of seven days of reduced violence and sustained access for humanitarian aid. Once the Russians and Americans agree that this has been achieved they will move toward joint targeting of groups they agree are terrorists.

Progress so far

Here’s the good news: Few casualties have been reported in the last 48 hours in areas of Syria not controlled by ISIS (those areas are excluded from this cessation of hostilities plan). So there is stuttering progress toward the seven days of quiet. Even if there is a major violation the deal won’t be abandoned. The clock will be “reset,” and efforts will continue to reach a continuous week of relative peace.

00:50 - Source: CNN
Moment of joy for Aleppo's children

Hitches in the road

Despite the decline in hostilities, the much-needed humanitarian relief to half a dozen areas under siege, principally rebel-held parts of Aleppo, has yet to materialize. The UN has not been able to supply the area, which has a population of some 250,000, since early July.

The route into Aleppo from the Turkish border, along the Castello Road, is yet to be declared safe. Part of the US-Russian plan is to demilitarize the area, according to a complex formula that measures the presence of rival groups to within a few hundred meters. But it’s not yet done.

Staffan de Mistura, the United Nations special envoy for Syria, said the regime had not followed through on pledges to provide permits to supply five other areas in severe need of aid, which he described as a “deeply regrettable” violation of the agreement.

Fighting has continued – albeit at a much lower level, with the regime and its opponents blaming each other for violations every day. There is no way to enforce the cessation on the ground, and no way to hold parties accountable for breaking it. Rebel groups and the regime suspect their enemies will exploit the pause to mobilize for new attacks or grab territory mechanisms for preventing such moves.

There is also plenty of rhetorical sniping between the US and Russia – even since the deal was reached a week ago.

Splitting the ‘moderates’ from the ‘terrorists’

This is the tough part. Once the humanitarian relief is in, the Russians and Americans are meant to agree on targeting jihadist factions: Jabhat Fateh Al-Sham, the former al Qaeda affiliate in Syria, and ISIS. To do that, they will set up a Joint Implementation Center (JIC).

The omens are not promising. The Russian Foreign Ministry is already complaining that the US is not keeping its pledge to distinguish between moderates and terrorists. Part of the agreement is that non-jihadist rebel groups in Syria must stop co-operating with and separate themselves from Jabhat Fateh al-Sham – until recently known as the al-Nusra Front – or face unspecified consequences.

But in many areas, including Aleppo and Deraa in the south, rebel “Operations Rooms” bring together Jabhat Fateh Al-Sham with other factions and even elements of the Free Syrian Army. Indeed, Jabhat Fateh Al-Sham is frequently the strongest component of any rebel offensive.

If it’s to be excluded from any deal, and then targeted once a period of sustained quiet occurs, Jabhat Fateh Al-Sham has no incentive to keep the peace, and every incentive to try to take territory while the cessation continues.

By the same token, the regime has the right – under the agreement – to target Jabhat Fateh Al-Sham until the JIC comes into force. That may well prove to be the Achilles heel of the US-Russian agreement.

Charles Lister, a Syria expert at Brookings who has extensive contacts with rebel groups, says that “many opposition figures see the US-Russia talks and whatever comes from them as a conspiracy against their long and hard fought-for revolution. It will be hard to change this mindset.”

US doubts

There are also a range of views among US officials about the agreement. The military is apparently more dubious about co-operating with the Russians. General Joseph Votel, who leads the US Central Command, said Wednesday: “We have to see how this goes first of all…whether it actually pans out or not, I don’t know.”

And Lt. Gen. Jeffrey L. Harrigian, commander of the US Air Forces Central Command, sounds skeptical about the JIC. “I’m not saying yes or no. It would be premature to say that we’re going to jump right into it,” he said Wednesday.

Conflicting aims

Even if the quiet persists, the US and Russia still have fundamentally different goals in Syria. The Russians doggedly support Bashar al-Assad, not because they think he’s the best President for Syria but because they fear a complete meltdown of the state if he were to be removed. And Syria is one of the few states in the region still in Moscow’s “sphere of influence.” US policy is to support “moderate” rebels and negotiate the removal of Assad, though who might replace him and how is a vexed question.

Turkey, Iran, Saudi Arabia and others also have stakes. Richard Haass, President of the Council on Foreign Relations and a veteran diplomat, says “there could well be too many players with conflicting agendas for a ceasefire to hold. Even major powers such as the US and Russia cannot impose an outcome.”

What if the deal holds?

What if aid starts getting through, the fighting abates and the Russians and Americans settle into their JIC? At that point, UN envoy Staffan de Mistura will begin thinking about reconvening peace talks between the different parties – but Jabhat Fateh Al-Sham would obviously be excluded.

The incentive for Moscow is that its military involvement in Syria, which Russian officials predicted would last a few months when it began a year ago, is still ongoing. The incentive for the US is that only Russia can coerce Assad into an agreement.

As Lebanese commentator Randa Slim, =”http://foreignpolicy.com/2016/09/12/can-anyone-stop-the-syrian-war-us-russia-ceasefire-spoilers/?utm_source=Sailthru&utm_medium=email&utm_campaign=flashpoints%20today&utm_term=Flashpoints&wp_login_redirect=0” target=”_blank”>puts it, “The question is whether or not this agreement can push Assad’s patrons to seriously entertain his exit from the political scene — and thus take a giant step forward to ending this war.”

But there are a lot of other questions before we reach that one.