Rights official says evidence points to high-level involvment in war crimes.
Marketplace near Aleppo struck in possible new push by govt forces.
Added that the international community must "prove its seriousness" by forcing Damascus to agree on trust-building measures.
World powers raised hopes for the first face-to-face talks to end the Syrian civil war as the United Nations called the warring parties to the table.
An al-Qaeda-linked group claimed responsibility for twin suicide bombings that struck outside the Iranian Embassy in Beirut on Tuesday, killing 23 people, including an Iranian diplomat.
Putin "emphasized efforts taken by Russia" on Geneva conference.
Lakhdar Brahimi appeared uncertain about prospects for the meeting.
Says measure is necessary in order for any political solution to succeed.
First direct contact since Lakhdar Brahimi made remark that Assad family's rule of Syria was "too long."
Al-Qaida-linked rebels seized control of the Christian town a week ago.
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