Syria's opposition has called for an international investigation into the twin bombings in Damascus that killed at least 55 people and injured around 370 on May 10.
In a statement, the Syrian Free Army called for investigators to determine who perpetrated the bombings.
Meanwhile, speaking to journalists in Tokyo, opposition Syrian National Council head Burhan Ghalioun said the government of President Bashar al-Assad was "now trying to kill" the peace plan put forward by UN-Arab League envoy Kofi Annan.
On May 10, UN Secretary-General Ban Ki-moon and the Security Council have condemned the deadly bomb blasts in the Syrian capital, Damascus.
Two suicide bombers detonated cars packed with explosives in Kazaz area of the capital, according to the government.
The attack, the deadliest in the 14 months since the Syrian uprising began, marked the worst violation of a United Nations-brokered cease-fire in effect since April 12.
Ban called on all sides to halt the violence and implement the peace plan -- including a cease-fire -- put forth by the international mediator, Kofi Annan.
"There is an urgent call on all sides fully to comply with their obligations to cease armed violence in all its forms, and to protect civilians, as well as to distance themselves from indiscriminate bombings and other terrorist acts," Ban's spokesman, Martin Nesirky, said.
"And let me just say of course, the secretary-general strongly condemns today's attacks in Damascus, which killed more than 50 people and injured scores of others."
The United States, Russia, and the European Union also condemned the attacks, but Moscow accused unspecified foreign countries of encouraging such violence.
U.S. Secretary of Defense Leon Panetta said the United States remained concerned over Syria and repeated calls for Assad to step down.
"The cease-fire does not appear to be working, and Annan himself has indicated concerns about whether or not parties are abiding by the cease-fire," Panetta said.
"There must be a change there," he added. "They've lost their legitimacy by the huge number of deaths that are taking place in Syria."
He added that U.S. intelligence indicated "an Al-Qaeda presence in Syria" but said the extent of the terrorist network's activity was unclear.
No one claimed responsibility for the bombings, but the government and the opposition blamed one another.
An opposition spokesman accused the Assad government of staging the attack in order to convince the international community it faces a terrorist threat.
The explosions targeted a military-intelligence building.
The attacks came a day after a bomb exploded near UN observers monitoring the cease-fire.
With reporting by Reuters, dpa, AFP, and AP