The Syrian government urged the U.N. Security Council to take action to combat terrorism, on Friday, a day after twin bombings killed at least 55 people in Damascus.

State media said the government made the plea in letters to the Security Council and U.N. chief Bank Ki-moon. It said “escalating crimes” were proof that Syria is facing terrorist attacks led by groups receiving foreign support.

Thursday's car bombings in the capital were the deadliest attacks in Syria since the anti-government uprising began 14 months ago.

In spite of the violence, U.N. observers monitoring a shaky cease-fire between the government and rebels toured Damascus on Friday.

Observer mission spokesman Neeraj Singh said the number of international monitors and staff members taking part in the mission had grown to 150.

“You have the world coming together, the world community coming together to be with the people of Syria to see in what way we can help. The most important thing being that violence in all its forms has to stop.”

Meanwhile, the head of Syria's main opposition group said al-Qaida-linked forces with ties to the Syrian government were responsible for Thursday's blasts.

In a Friday news conference from Tokyo, Burhan Ghalioun said the government was trying to sabotage the peace plan brokered by international envoy Kofi Annan.

There was no immediate claim of responsibility for the blasts, which further shredded the April 12 cease-fire declared by Mr. Annan.

U.S. Defense Secretary Leon Panetta told reporters U.S. intelligence indicates “an al-Qaida presence in Syria,” but said the extent of its activity is unclear.

The United Nations says more than 9,000 people have been killed in violence related to the anti-government uprising which erupted in March 2011.