Rebel factions in Syria expressed deep reservations on Sunday about the terms of a U.S.-Russian deal that seeks to restart the peace process for the war-torn country, with the leader of at least one U.S.-backed rebel faction publicly calling the offer a "trap."

The second in command of the powerful, ultraconservative Ahrar al-Sham group condemned the superpower agreement as an effort to secure President Bashar Assad's government and drive rebel factions apart.

"A rebellious people who have fought and suffered for six years cannot accept half-solutions," said Ali al-Omar in a video statement.

But the commander and other rebel leaders stopped short of fully rejecting the agreement's interim cease-fire, which is slated to come into effect in stages beginning on Monday at sunset.

The deal hammered out between U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov in Geneva Saturday allows the Syrian government to continue to strike at al-Qaida-linked militants, until the U.S. and Russia take over the task in one week's time.

The arrangement has divided rebel factions, who have depended on the might of the powerful al-Qaida-linked Jabhat Fatah al-Sham faction to resist government advances around the contested city of Aleppo.

Al-Omar said his group would "refuse the targeting of any faction of our blessed factions" and called on rebels to unify into a single front.

Still, a senior official inside Ahrar al-Sham said rebels would nevertheless abide by the cease-fire to regroup after a punishing conflict with pro-government forces over Aleppo.

"The Islamist factions and Jabhat Fatah al-Sham will abide by the cease-fire without publicly declaring it," said the official. "They will announce they are opposed to the U.S.-Russian agreement, but they will halt their operations on the ground because of the losses they sustained in the battles for Aleppo," he said, speaking on condition of anonymity.

Other factions less closely tied to Jabhat Fatah al-Sham, including those backed by Turkish ground forces in the northern frontier area, will publicly commit to the agreement, according to the Ahrar al-Sham official.

"The free Syrian factions under the Euphrates Shield banner will announce their commitment to the agreement, of course," he said.

Under the terms of the agreement, the U.S. and Russia will coordinate to target the Islamic State group in Syria and Jabhat Fatah al-Sham, while rebels and the Syrian government will be expected to stop attacking one another. The deal has received the endorsement of President Bashar Assad's government and its key allies — Russia, Iran and the Lebanese militant group Hezbollah.

But that scenario is complicated by the fact that Jabhat Fatah al-Sham remains intertwined with several other factions. It is not clear how these governments intend to distinguish between Jabhat Fatah al-Sham and other allied rebel factions or how they will be able to attack the al-Qaida linked militants without hitting other rebels as well.

Despite fundamental differences in their vision for Syria, rebels and opposition activists hailed a rebel coalition led by Jabhat Fatah al-Sham when it broke a government siege on the rebel-held eastern quarters in Aleppo. The U.N. estimated a quarter million residents were trapped inside with dwindling food and medical supplies.

The government has since re-established its siege.

Over 2,000 people have been killed in fighting over the past 40 days in Aleppo, including 700 civilians and 160 children, according to a Syrian human rights group. One of the more immediate goals of the Kerry-Lavrov agreement is to allow the U.N. to establish aid corridors into Aleppo.

On Saturday, presumed Russian or government airstrikes on rebel-held Idlib and Aleppo provinces killed over 90 civilians, including 13 children in an attack on a marketplace in Idlib, according to the Britain-based Syrian Observatory for Human Rights.

In the aftermath on Sunday, rebels and opposition activists were asking whether the government's side could be trusted.

"What truce, when the regime commits a massacre in Idlib?" said Ahmad Saud, commander of the U.S.-backed Division 13 brigade, on Twitter. "I am starting to feel that the truce is a military trap to kill us more."

Several previous negotiated cease-fires have all eventually collapsed. A partial "cessation of hostilities" that brought sorely needed relief to civilians in March unraveled as the government continued to strike targets in opposition areas, including near a hospital and school near Damascus and a marketplace in Idlib province, killing dozens of civilians.

Previous cease-fires were also preceded by soaring violence as parties on all sides sought to improve their positions in the build-up.