Syrian regime forces shelled two central Damascus districts Wednesday before troops backed by tanks swept through to carry out house-to-house raids, killing at least 35 suspected rebels, in a major flare-up of fighting in the Syrian capital, activists said.

In a particularly hard-hit northern district, activists said they discovered dozens of bodies that appeared to have been shot execution-style. Such gruesome reports have become increasingly common in recent months as the civil war has taken on heavy sectarian undertones.

The capital is one of many fronts President Bashar Assad's regime is struggling to contain as the 17-month-old rebellion against his rule gains strength.

Government forces are also engaged in a major battle for control of the northern city of Aleppo as well as smaller scale operations in the country's south, east and center.

On the diplomatic front, a senior U.N. official said Iran's arms supplies to Syria violated U.N. sanctions. France also indicated it has provided the rebels with communication and protection equipment but cautioned against foreign intervention without a U.N. mandate.

A prominent opposition figure, meanwhile, rejected as "more lies" comments by a senior Syrian official that Damascus would be willing to discuss Assad's resignation but only after the opposition agreed to join in negotiating a peaceful settlement.

"As for his resignation, making his resignation a condition for dialogue effectively makes holding such a dialogue impossible," said the official, Deputy Foreign Minister Qadri Jamil. "During the negotiating process any issues can be discussed, and we are ready to discuss even this issue."

Reached in Turkey, Adib Shishakly of the Syrian National Council, a key umbrella opposition group, said: "It's the first time that we hear such talk, but it's difficult to believe. We have grown accustomed to the regime's lies."

Around dawn Wednesday, regime forces in Damascus rained mortar shells on the upscale Kafar Soussa area — home to the foreign ministry, the prime minister's office and several foreign embassies — and adjacent Nahr Eishah, activists said.

Government troops appeared to be shelling the districts from the Qasioun mountain overlooking the capital, a Damascus resident said on condition of anonymity for fear of retribution.

The attacks may have been designed to kill or capture rebel mortar teams who have used the two neighborhoods in recent days to target the city's strategically located Mazzeh military airport, activists said.

The British-based Syrian Observatory for Human Rights said at least 24 people were killed in Kafar Soussa on Wednesday and fierce battles were raging in an area just outside the neighborhood.

An activist in Kafar Soussa reached on Skype corroborated the Observatory's findings. He also spoke on condition of anonymity because he feared reprisals. The reports could not be independently verified.

Earlier, an activist who only wanted to be identified by the name Bassam for fear of retribution, said as many as 22 tanks stormed Kafar Soussa with about 20 soldiers on foot behind each one. He spoke via Skype from central Damascus.

Bassam and the Observatory also reported heavy government shelling of Nahr Eishah early Wednesday. They said regime forces then searched houses for rebels. Bassam said as many as 12 people were killed in Nahr Eishah, while the Observatory put the death toll at eight, saying they were all men shot dead by troops.

Amateur videos posted by activists on the Internet showed tanks rumbling into Nahr Eishah, kicking up a cloud of dust as the sound of gunfire is heard in the background. The authenticity of the videos could not be verified.

It was not clear whether those killed in the two areas died in the shelling or later government raids. Activists, including the one reached by Skype in Kafar Soussa, spoke of execution-style killings in both areas.

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.