As many as 46 bodies of people shot execution-style were found in the northern Damascus district of Qaboun, the Local Coordination Committees activist network said. The Observatory reported "dozens of bodies who appeared to have been shot at close range" there.
Syria's civil war has its roots in a mostly peaceful uprising against Assad's regime that began in March last year. The uprising grew increasingly violent as the government launched a brutal crackdown on protesters, prompting many to take up arms to forcefully overthrow Assad's regime.
The conflict has defied all international efforts to end it. At least 20,000 people have been killed, according to human rights activists.
The U.N.'s political chief Jeffrey Feltman said Iran's delivery of weapons to the Syrian government was an apparent violation of U.N. sanctions banning arms exports by Tehran. He raised the issue in Wednesday's monthly Mideast briefing to the U.N. Security Council.
Feltman said the Syrian government and opposition were focusing on the use of force, with the government using heavy weapons on populated areas and the Syrian people "suffering grievously from the appalling further militarization of this conflict."
Iran, together with Russia and China, are the Syrian regime's strongest backers.
On Wednesday, Russia's Foreign Ministry accused Western powers of "openly instigating" Syrian opposition groups to take up arms against Assad's regime. The West, it said, "has done nothing" to urge the Syrian opposition to start a dialogue with the government. "Instead, they are engaged in openly instigating it to continue their armed struggle," the ministry said in a statement.
Moscow, which wields veto power on the U.N. Security Council, has shielded Assad's regime from international sanctions and provided it with weapons despite an international outcry.
In Paris, French Prime Minister Jean-Marc Ayrault —a leading participant in NATO's air campaign last year in Libya — cautioned that his country opposed any military intervention in Syria without a U.N. mandate. However, he indicated that France has provided communication and protection equipment to Syrian rebels.
"We cannot abandon the Syrian people. We have sent in some non-lethal means, we are not impassive. And you see things are moving," Ayrault said on France's BFM TV.
In Brussels, the European Union's humanitarian aid commissioner urged the U.N. Security Council to unite in the face of what she said was an escalating humanitarian disaster in Syria and ensure that assistance reaches civilians affected by the civil war.
"Over the last weeks, the humanitarian condition in Syria ... is going from bad to terrible," Kristalina Georgieva said. "Today we have 2.5 million inside Syria in need of assistance, among them 1.2 million displaced population and the number of refugees in Lebanon, Jordan, Turkey, is growing by the day."
Associated Press correspondents Edith M. Lederer at the United Nations, Slobodan Lekic in Brussels and Jamey Keaten in Paris contributed to this report.