It was not immediately known if the vote would be postponed on a draft resolution demanding a truce to allow humanitarian aid deliveries and the evacuation of the sick and wounded.
US Ambassador Nikki Haley told reporters as she went into the council chamber: "Today we are going to see if Russia has a conscience."
Russia is supporting the Syrian government, and the UN negotiations have stumbled over Russian demands that rebel groups fighting government forces comply with the truce.
Negotiations on Friday led to amendments to the proposed measure, which now states that the ceasefire will begin "without delay," not after 72 hours as provided in a previous draft.
The text demands the lifting of all sieges including in Eastern Ghouta, where Syrian government forces are waging a fierce bombing campaign.
The vote, initially expected on Thursday, was delayed to Friday and finally rescheduled for Saturday.
Sweden and Kuwait presented the measure two weeks ago.
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