(CNN) Syrian rebels are suspending talks in the lead-up to planned peace negotiations due to what they call "Syrian regime violations" of a four-day-old nationwide ceasefire agreement.
The Free Syrian Army (FSA), an association of 12 rebel groups, has not withdrawn from the actual peace talks yet -- brokered by Russia and Turkey and slated to take place in late January -- but the announcement is a sign of the obstacles that lie ahead.
The FSA and other armed groups say the regime is continuing its siege of rebel-held areas outside Damascus -- and claim government forces have launched an assault on Wadi Barada, which supplies the capital with much of its water.
On the other side of the country, a rebel group in the northwest, Suqour al-Sham, says it has fired on regime positions near Idlib in response to violations.
The opposition fears the regime is using the truce to regroup and selectively pick off rebel-held areas it wants to regain. But the regime says it's going after "terrorists" who were deliberately excluded from the agreement.
MORE: Water is the latest battleground in Syria
In Wadi Barada, for example, it alleges that the former al Qaeda affiliate, Jabhat Fateh al-Sham, is present (although local groups deny this). The exclusion of groups like Fateh al-Sham is the Achilles heel of the ceasefire.
There is no trust in the process, nor is it monitored or enforced by any neutral party, even though Russia and Turkey have set themselves up as guarantors. The FSA says "several requests were made to the guarantor of the regime [Russia] and its allies to stop these breaches," but that they had continued - including forced evacuations.
The FSA wants any gains made by the regime since the truce was announced to be reversed - and in its statement says: "The inability of the guarantor to impose the terms of the agreement makes us wonder of its ability to impose any other obligations" on Syria's government.
Politically, yes.
After the fanfare in Russia and Turkey about how many groups had signed up to the new three-part arrangement (truce, monitoring and then peace talks), any withdrawal from the talks due to take place in Astana, Kazakhstan would be embarrassing. But it wouldn't be the first time negotiations on the Syrian conflict have been postponed or boycotted.
Militarily, it matters less. The rebel groups are divided among themselves on how to respond to the Russian-Turkish initiative. After the fall of Aleppo, the regime and its allied militia (as well as Russian airpower) hold the whip-hand.
Turkey controls the main weapons pipeline for rebels, while Saudi Arabia and Qatar, the sponsors of several Islamist groups, seem for now outflanked.
If the regime manages to starve out a few more rebel-held areas, it would be in an even stronger position to dictate terms of a settlement.
There are some significant groups, including:
And some notable absentees:
So some big players who can impact Syria's future as a state are excluded. The position of the Kurds is the most intractable. Historically they have been supported by Moscow, and they are also supported by the US as a force against ISIS. But the YPG is regarded by Turkish authorities as a terrorist group.
Not enough, according to various rebel representatives.
One problem is the sheer number of 'fronts' across Syria involving different groups, many of whom have collaborated with Jabhat Fateh al-Sham. Provocations and violations can't be monitored or punished.
Read: How Russia and Turkey brokered peace in Syria -- and sidelined the US
Turkey has influence over some FSA and Islamist factions, but is also preoccupied with the anti-ISIS offensive it is supporting around al-Bab, and attacks at home.
Russia has great influence with the regime's forces, but less with the plethora of militia that support them, from Syrian irregulars known as 'shabiha,' to Iraqi and even Afghan Shia paramilitaries.
Hezbollah, a critical element in the regime's camp, looks to Iran for guidance. It's been involved in the assault on Wadi Barada outside Damascus. Human rights groups have described the shabiha as a law unto themselves - responsible for summary executions and mass rape.
The truce was specifically negotiated by Russia and Turkey. The US has influence with a few (though diminishing) groups in Syria -- but none with the regime, Hezbollah or Islamist opposition groups.
In turn most moderate Syrian elements have distanced themselves from the US, regarding the Obama Administration's support as utterly inadequate and expecting the incoming Trump Administration to work more closely with Russian President Vladimir Putin.
During this interregnum, Washington doesn't have a dog in the fight, unless you include the YPG, which it's wary of supporting more openly for fear of further infuriating Turkey.