GAZA CITY (Ma'an) -- PLO factions on Friday condemned the deadly bombing attack in Damascus a day earlier that left dozens dead and hundreds wounded.

Two suicide car bombers killed 55 people and wounded 372 in the Syrian capital on Thursday, state media said, the deadliest attacks in the Syrian capital since an uprising against President Bashar Assad began 14 months ago.

In a statement, PLO parties said the "act of terrorism" targeted Arab and Syrian national unity.

"We strongly condemn this criminal act, and we call on all Syrian people to promote their national unity."

The White House and the United Nations also condemned the attacks, for which there was no claim of responsibility.

Syria's foreign ministry said the attacks were a sign that the Arab state was facing foreign-backed terrorism and urged the UN Security Council to combat countries or groups supporting such violence.

The opposition Syrian Observatory for Human Rights put the death toll from the bombings at 59 and said most of them worked for the security forces. No group has claimed responsibility.

The White House said it did not believe the attacks were representative of the opposition to Assad, contrary to what the Syrian authorities and state media have suggested.

"There are clearly extremist elements in Syria, as we have said all along, who are trying to take advantage of the chaos in that country, chaos brought about by Assad's brutal assault on his own people," White House spokesman Jay Carney said.

Samir Nashar, a member of the opposition Syrian National Council's executive board, blamed the state for the bombings, saying they were meant to deter protesters and UN monitoring, an argument echoed by rebel Free Syrian Army leaders.

"These bombs are not the work of opposition fighters," said its chief, General Mustafa al-Sheikh, adding that the FSA lacked the capability to set off such big explosions.

Reuters contributed to this report