MOSCOW, Sept 12 (Reuters) - The Russian Foreign Ministry said on Monday it was concerned that some armed opposition groups in Syria including the powerful insurgent group Ahrar al-Sham have refused to abide by a ceasefire agreement that took effect at sunset.

The ministry also said in a statement that it was counting on the United States to influence Syria's "moderate opposition" to ensure full compliance with the cessation of hostilities that began at 1600 GMT on Monday.

"We count on the US to deliver its part and use the necessary influence on those it considers 'moderate opposition'," the ministry said.

It also said humanitarian aid to Syria's Aleppo would begin immediately through the northern road of Castello. A southern road through the region of Ramusa will open "over time." (Reporting by Lidia Kelly)

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