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Based on its investigation, the team said in a statement that “an attack” took place Thursday. It said the violence seemed to target the homes of army defectors and activists, some of which were burned or damaged and had pooled or splattered blood and bullet casings inside.

Although much of the international community has turned on Mr. Assad, Damascus still has some key allies — including Russia and Iran. The Kremlin announced Sunday that Mr. Annan will meet Russian President Vladimir Putin on Tuesday.

Also Sunday, Iranian Foreign Minister Ali Akbar Salehi said Tehran is ready to invite Syrian opposition groups and government envoys for talks, the semiofficial ISNA news agency reported.

Any proposal from Iran is likely to be rebuffed by rebel groups, which have rejected negotiations with Mr. Assad’s government and have criticized Tehran for standing by its allies in Damascus. But the offer suggested that Iran is seeking a more active role in mediation efforts following a visit last week to Tehran by Mr. Annan, who is seeking to keep alive his flagging peace efforts.