Ceasefire in Syria? Rebels agree but not without reservations.

Syrian rebel groups agreed to observe a ceasefire set to begin Monday, despite having reservations about details of the deal. 

The ceasefire plan, announced Friday by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, is set to take effect at sundown Monday. If successful, a lasting ceasefire could be a "turning point" in the five-year civil war between Syrian President Bashar al-Assad's forces and rebel groups, Mr. Kerry said. 

The ceasefire plan excludes extremist groups, such as the Islamic State. A previous ceasefire attempt this year collapsed, but if this one holds, Russia and the United States will share intelligence and cooperate against extremists, ABC reports. 

In a letter to the US on Sunday, Free Syrian Army rebel groups wrote, "We in the revolutionary factions reconfirm our fixed position of dealing positively with the idea of a ceasefire." At the same time, the rebels wrote, they had reservations "linked to our survival and continuation as a revolution" regarding some conditions of the agreement. 

Although the letter did not explicitly say the groups would lay down their weapons, rebel representatives told Reuters that their groups would respect the terms of the ceasefire. 

One concern cited in the letter was the lack of monitoring mechanisms or sanctions against violations, as rebels worried that the government could thwart the deal with no repercussions. The letter also expressed concern over clauses in the agreement indicating that Syrian government jets wouldn't be barred from flying for up to nine days after the ceasefire began.

"As you know this time frame ... will give the warplanes of the regime adequate opportunity to cause huge damage in civilian areas and against our military forces," the letter said.

Rebel groups also argued that excluding jihadist groups Jabhat Fateh al-Sham and the self-proclaimed Islamic State from the deal, but not excluding foreign militias backed by Iran, set a double standard, and could provide Russia and the Syrian government with a pretext to attack other rebel groups.

Whether the ceasefire is a success will depend largely on Aleppo, the city where much of the fighting has taken place. Recent battles over Aleppo could have significant implications, as The Christian Science Monitor’s Howard LaFranchi reported last week: 

The outcome, many regional experts say, likely won't end the complex and devastating war that has cost more than 300,000 Syrian lives, displaced millions, and spawned an epic flow of humanity to European shores. But it is likely to determine both the course of the war and the political prospects of both the regime and Syria’s opposition.

'If the regime is unable to retake Aleppo, that will demonstrate that they are unable to retake all of Syria,' says Andrew Tabler, an expert on Syria and United States policy at the Washington Institute for Near East Policy. 'If the opposition suffers defeat and is routed from the city, it’s a sign the revolution has lost.'

In the letter, the rebels expressed concerns that the ceasefire agreement focused only on Aleppo while ignoring other areas under siege, and did not guarantee that the government would not impose local agreements on these affected areas. Therefore, they wrote, "we insist on the necessity of aid going in to all the besieged areas without exception." 

This report includes material from Reuters.