(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The Free Syrian Army (FSA), an association of 12 rebel groups, has not withdrawn from the actual peace talks yet -- brokered by Russia and Turkey and slated to take place in late January -- but the announcement is a sign of the obstacles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.

Why are rebels threatening to boycott talks?

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.

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.

Does it matter if the rebels don't show up for talks?

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.

How powerful are the rebels who were invited to the talks?

There are some significant groups, including:

Ahrar al Sham, though it has not agreed to take part and appears to be split

Jaysh al Fatah (the Army of Conquest), a shifting alliance which has been a player in Idlib and Aleppo

Suqour al-Sham, which fired on regime positions near (NEAR) Idlib Monday

Turkish-backed FSA elements, which are now battling ISIS north of Raqqa

And some notable absentees:

Jabhat Fateh al-Sham (formerly Jabhat al-Nusra), which has endured despite massive bombardment of Idlib and Aleppo

ISIS, which still controls large parts of northeastern Syria, its self-declared capital of Raqqa, and is active elsewhere

The Kurdish YPG, which controls a swathe of northern Syria bordering Turkey

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.

How much control do Turkey and Russia have over the sides they're backing?

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.

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.

Photos: The battle for Aleppo in 20 photos Photos: The battle for Aleppo in 20 photos A Syrian man reacts while standing on the rubble of his house while others look for survivors and bodies in the Tariq al-Bab district of the northern city of Aleppo on February 23, 2013. Hide Caption 1 of 20 Photos: The battle for Aleppo in 20 photos A Free Syrian Army fighter aims his weapon during clashes with government forces in Aleppo on Tuesday, January 15, 2013. Hide Caption 2 of 20 Photos: The battle for Aleppo in 20 photos A member of the Syrian pro-government forces stands amid heavily damaged buildings in Aleppo's 1070 district on November 8, 2016, after troops seized it from rebel fighters. Hide Caption 3 of 20 Photos: The battle for Aleppo in 20 photos A wounded Syrian boy cries after bombs fell on the opposition-controlled Firdevs neighborhood in Aleppo on October 11, 2016. Hide Caption 4 of 20 Photos: The battle for Aleppo in 20 photos Smoke rises after a bomb explodes in a residential area in the Darat Izza neighborhood of Aleppo on October 4, 2016. Hide Caption 5 of 20 Photos: The battle for Aleppo in 20 photos Search and rescue team members carry an injured man after Syrian regime airstrikes targeted the Meshed neighborhood of Aleppo on July 21, 2016. Hide Caption 6 of 20 Photos: The battle for Aleppo in 20 photos A rebel fighter aims his weapon toward Syrian government forces' positions at the Menagh military airport near Aleppo on March 13, 2013. Hide Caption 7 of 20 Photos: The battle for Aleppo in 20 photos The body of a Syrian army soldier lies on the ground after heavy clashes with government forces at a military academy besieged by the rebels in Tal Sheer village, north of Aleppo, on December 16, 2012. Hide Caption 8 of 20 Photos: The battle for Aleppo in 20 photos Debris covers a street and flames rise from a building after a reported airstrike by Syrian government forces on March 7, 2014, during the Friday prayer in the Sukkari neighborhood of Aleppo. Hide Caption 9 of 20 Photos: The battle for Aleppo in 20 photos Wounded 5-year-old Omran Daqneesh sits alone in the back of an ambulance after he was injured during a Russian or Assad regime forces airstrike targeting the Qaterji neighborhood of Aleppo on August 17, 2016. Hide Caption 10 of 20 Photos: The battle for Aleppo in 20 photos A Syrian opposition tank fires a rocket toward an Assad forces' building during clashes near the Air Intelligence building of Jamiat al-Zahra, Aleppo, on April 13, 2015. Hide Caption 11 of 20 Photos: The battle for Aleppo in 20 photos On September 7, 2012, Free Syrian Army fighters run after attacking a Syrian army tank during fighting in the Izaa district of Aleppo. Hide Caption 12 of 20 Photos: The battle for Aleppo in 20 photos Search and rescue team members inspect collapsed buildings after Assad regime forces attacked residential areas in the Karm al-Beik region of Aleppo on July 9, 2015. Hide Caption 13 of 20 Photos: The battle for Aleppo in 20 photos Syrian civil defense volunteers and rescuers remove a baby from under the rubble of a destroyed building after a reported airstrike on the rebel-held neighborhood of al-Kalasa in Aleppo on April 28, 2016. Hide Caption 14 of 20 Photos: The battle for Aleppo in 20 photos A Free Syrian Army fighter takes cover during clashes with Syrian army soldiers in the Salaheddine neighborhood of central Aleppo on August 7, 2012. Hide Caption 15 of 20 Photos: The battle for Aleppo in 20 photos Syrian pro-government forces walk in the damaged ancient Umayyad Mosque in the old city of Aleppo on December 13, 2016, after they captured the area. Hide Caption 16 of 20 Photos: The battle for Aleppo in 20 photos A man in front of a field hospital mourns the death of his relatives on August 21, 2012, in Aleppo. Hide Caption 17 of 20 Photos: The battle for Aleppo in 20 photos Rubble is seen in the Salaheddine neighborhood on March 24, 2013, in Aleppo. Hide Caption 18 of 20 Photos: The battle for Aleppo in 20 photos Syrian government forces walk in the strategic area of the Bazo hilltop, on the southern outskirts of Aleppo, as they advance to seize the rebel-held eastern part of the city on October 25, 2016. Hide Caption 19 of 20 Photos: The battle for Aleppo in 20 photos An aerial view shows a convoy of buses and ambulances waiting at a crossing point in the Amiriyah district of Aleppo on December 15, 2016, to evacuate civilians trying to flee from areas under siege by Iran-led Shiite militias and Assad regime forces. Hide Caption 20 of 20

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.

Why isn't the US involved?

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.

Posters of Syrian President Bashar al-Assad are seen in front of shop front's in Damascus on January 3rd.

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.