Syria's foreign ministry underscored the urban violence in identical letters to U.N. Secretary-General Ban Ki-moon and U.N. Security Council President Agshin Mehdiyev, who is the Azerbaijani ambassador to the United Nations.

The letters said that Damascus, Aleppo and other Syrian cities "witnessed several terrorist bombings over the last weeks" that left dozens of innocent people dead, the state-run news agency reported.

The letters said the country is facing "terrorist" assaults spearheaded by groups that get arms and money from entities encouraging the actions.

The letters said the "armed terrorist groups" are violating international envoy Kofi Annan's peace plan and attacked a convoy from the U.N. observer team Wednesday in Daraa province. There were no injuries in that bombing attack. Maj. Gen. Robert Mood, the head of the observer team, was in the convoy.

"Syria will move forward to combat terrorism and defend its people and sovereignty and preserve security and stability in it," the letters said.

Syria blames "terrorists" for the attacks, the term it uses to describe the opposition and rationalize security forces' crackdown. Some analysts said the attacks raise concerns about the presence of jihadist elements in Syria, noting Thursday's Damascus strikes resemble suicide car bombings during the sectarian violence in Iraq in the past decade.

But opposition groups have said the regime is responsible for the violence that erupted after it began a crackdown on peaceful protests in March 2011. That fierce clampdown spurred a grassroots uprising against the regime.

The United Nations estimates that at least 9,000 people have died in the conflict, while opposition groups put the death toll at more than 11,000.

The opposition Syrian National Council said al-Assad's regime staged Thursday's deadly suicide bombings in Damascus "to spur chaos, disrupt the work of the international observers and divert attention away from other crimes being committed by its forces elsewhere."

"In orchestrating such acts," the council said Friday, "the regime seeks to prove its claims of the existence of 'armed terrorist gangs' in the country that are hindering its so-called 'efforts of political reform.' "