(CNN) -- Shelling hit areas near two key cities in eastern Ukraine on Sunday morning, intensifying fears that a ceasefire that took effect less than two days ago may be falling apart. Why is the ceasefire under strain? A variety of fighting factions in the conflict zone -- on both sides -- may not fall directly under a military chain of command. The pro-Russian rebels are mostly volunteer militias; fighting against them on the Ukrainian side are at least some far-right nationalist militias. Controlling these groups is difficult and some may have different aims, including sabotaging the truce. At this point it's been nearly impossible to figure out who's doing the firing and why. The conditions of the ceasefire agreement don't help either. The conditions are vague and at this point there doesn't seem to be an effective mechanism in place inside the conflict zone to monitor and enforce the agreement. Why can't the two sides' leaders control their forces? It's unclear if Kiev has control over all of the fighting forces in eastern Ukraine. Some of the volunteer militias fighting alongside Ukrainian soldiers are far-right nationalists who've been critical of the current government in Kiev, but they're still fighting because they feel Ukraine is under attack by Russia. And who controls the pro-Russian rebels? Is it the local commanders? Is it Russian President Vladimir Putin? None of that is clear. Which side has the most to gain from the truce? If the truce leads to good-faith negotiations and a compromise, then both sides can gain. A compromise could look like something like this: The pro-Russian region of Donbas gets autonomy and self-determination under a federalized Ukrainian government, and in return the rebels drop their demand for independence and Kiev gets to protect Ukraine's sovereignty and territorial integrity. There are elements on both sides that don't want a compromise, and they could certainly have the potential of undermining the truce. What happens next? We wait to see if the overnight shellings and firings are an anomaly or if they're a sign of more violence and more fighting. If the ceasefire sticks, both sides have agreed to hold talks that will address the core issues and demands on both sides that are still unresolved -- including the disarming of the rebels, a guarantee of self-determination for the pro-Russian Donbas region, the fate of Russian-annexed Crimea, constitutional reform, and a solution to the humanitarian crisis in the conflict zone. Obviously if the fighting continues, all bets are off. What effect does this have on the rest of the world? What happens in the coming days will determine the next move by NATO and Western leaders. If the ceasefire falls apart, the West will likely turn up the pressure by following through with sanctions and bolstering Western forces in NATO's Baltic-member states. Moscow has already threatened to respond if that happens. The bottom line is, the conflict will escalate and so will the prospects of a regional conflict -- although at this point that seems unlikely.
Shelling in eastern Ukraine raises fears that a ceasefire may fall apart .
Reza Sayah: It's been nearly impossible to say who's doing firing and why .
It's unclear who has control over fighting forces on either side in east Ukraine .
There are elements on both sides that don't want a compromise .
(CNN) -- Shelling hit areas near two key cities in eastern Ukraine on Sunday morning, intensifying fears that a ceasefire that took effect less than two days ago may be falling apart. Why is the ceasefire under strain? A variety of fighting factions in the conflict zone -- on both sides -- may not fall directly under a military chain of command. The pro-Russian rebels are mostly volunteer militias; fighting against them on the Ukrainian side are at least some far-right nationalist militias. Controlling these groups is difficult and some may have different aims, including sabotaging the truce. At this point it's been nearly impossible to figure out who's doing the firing and why. The conditions of the ceasefire agreement don't help either. The conditions are vague and at this point there doesn't seem to be an effective mechanism in place inside the conflict zone to monitor and enforce the agreement. Why can't the two sides' leaders control their forces? It's unclear if Kiev has control over all of the fighting forces in eastern Ukraine. Some of the volunteer militias fighting alongside Ukrainian soldiers are far-right nationalists who've been critical of the current government in Kiev, but they're still fighting because they feel Ukraine is under attack by Russia. And who controls the pro-Russian rebels? Is it the local commanders? Is it Russian President Vladimir Putin? None of that is clear. Which side has the most to gain from the truce? If the truce leads to good-faith negotiations and a compromise, then both sides can gain. A compromise could look like something like this: The pro-Russian region of Donbas gets autonomy and self-determination under a federalized Ukrainian government, and in return the rebels drop their demand for independence and Kiev gets to protect Ukraine's sovereignty and territorial integrity. There are elements on both sides that don't want a compromise, and they could certainly have the potential of undermining the truce. What happens next? We wait to see if the overnight shellings and firings are an anomaly or if they're a sign of more violence and more fighting. If the ceasefire sticks, both sides have agreed to hold talks that will address the core issues and demands on both sides that are still unresolved -- including the disarming of the rebels, a guarantee of self-determination for the pro-Russian Donbas region, the fate of Russian-annexed Crimea, constitutional reform, and a solution to the humanitarian crisis in the conflict zone. Obviously if the fighting continues, all bets are off. What effect does this have on the rest of the world? What happens in the coming days will determine the next move by NATO and Western leaders. If the ceasefire falls apart, the West will likely turn up the pressure by following through with sanctions and bolstering Western forces in NATO's Baltic-member states. Moscow has already threatened to respond if that happens. The bottom line is, the conflict will escalate and so will the prospects of a regional conflict -- although at this point that seems unlikely.
fears that a ceasefire that took effect less than it's been nearly impossible to figure out who's doing the firing and why. The conditions of the leaders control their forces? It's unclear if Kiev has control over all of the fighting forces in eastern Ukraine. Some of the volunteer territorial integrity. There are elements on both sides that don't want a compromise, and they