The Latest on the conflict in Syria and the provisional cease-fire proposed by the U.S. and Russia (all times local):

12:30 p.m.

A spokesman for a Saudi-backed alliance of Syrian opposition and rebel factions says the group has "major concerns" that Russia and the Syrian government will continue to strike at mainstream rebels under the pretext of hitting "terrorist groups" during the truce that is to go into effect later this week.

Salem Al Meslet says the alliance known as the High Negotiations Committee is holding open meetings in the Saudi capital of Riyadh and is seeking guarantees and clarifications from the United States about the mechanism for the implementation of the agreement.

He says however that the opposition wants to stop the bloodshed and would abide by the truce. Al Meslet spoke Wednesday in a phone interview with The Associated Press.

The agreement, engineered by the U.S. and Russia, is set to take effect at midnight Friday local time.

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11:50 a.m.

Syria's state-run news agency says Syrian President Bashar Assad has received a phone call from Russian leader Vladimir Putin.

SANA says that during the call with Putin on Wednesday, the Syrian president confirmed Damascus' readiness to support the implementation of the agreement for a "cessation of hostilities" in Syria.

The agreement, engineered by the U.S. and Russia, is set to take effect at midnight Friday local time. It does not cover the Islamic State group, Syria's al-Qaida branch known as the Nusra Front, or any other militia designated as a terrorist group by the U.N. Security Council.

SANA says the two leaders stressed the importance of continuing to fight the Islamic State, the Nusra Front "and other terrorist organizations."