ALGIERS, Algeria (CNN) -- Rescuers are sifting through the rubble of the United Nations headquarters in Algiers hoping to find survivors after a powerful bomb ripped off the building's facade and leveled nearby U.N. offices.
Rescuers and bomb experts search for survivors in the rubble of a destroyed building.
It was one of two suspected car bombs that struck Algiers within 10 minutes of each other.
The death toll is unclear: the official government count is at least 26, but hospital sources in Algiers told CNN affiliate BFM-TV that 76 people were killed in the two blasts. A statement from the United Nations said 45 people were reported killed.
Algerian Interior Minister Noureddine Yazid Zerhouni blamed a militant Islamic group with ties to al Qaeda for the attacks, which also targeted a building housing Algeria's Constitutional Council and Supreme Court.
In a posting on an Islamist Web site, the group al Qaeda Islamic Maghreb claimed responsibility.
CNN could not immediately corroborate that claim, but the Web site is known to carry messages, claims and videos from al Qaeda and other militant groups.
In the posting, the bombers were identified as Sheikh Ibrahim Abu Othman and Abdel Rahman Abu Abdel Nasser al-Asimi. It said two trucks were filled with "no less than 800 kg (1,763 pounds) of explosives."
The group called the operation "another successful conquest and a second epic that the knights of faith have dictated with their blood, defending the wounded Islamic nation and in defiance to the Crusaders and their agents, the slaves of America and the sons of France."
At least 10 U.N. staffers were among those killed, according to U.N. spokeswoman Marie Okabe.
The offices of the UN High Commissioner for Refugees -- located across the street from the U.N. headquarters -- were leveled by a blast that struck about 9:30 a.m. (3:30 a.m. ET) Tuesday.
"Our offices are basically destroyed now, nothing works," UNHCR spokesman Ron Redmond said from its Geneva headquarters. Watch his full interview
He said rescuers are working into the night trying to get to the trapped U.N. workers. "It's a very serious situation still with the U.N. in Algiers," he said.
In a strongly worded statement, U.N. Secretary-General Ban Ki-moon condemned what he called "an abjectly cowardly strike against civilian officials serving humanity's highest ideals under the U.N. banner."
"The perpetrators of these crimes will not escape the strongest possible condemnation -- and ultimate punishment -- by Algerian authorities and the international community," Ban said in the written statement.
He said he has sent senior advisers and other top U.N. officials to head to Algiers to assist in the investigation and rescue effort.
Most of those killed in the coordinated attacks were victims of the first suspected car bombing near the Constitutional Council -- which oversees elections -- and Supreme Court in the Algiers neighborhood of Ben Aknoun, according to the state-run Algeria Press Agency.
That blast struck a bus outside the targeted building, killing many of those on board, the news agency reported.
One man said he heard the first blast then the second exploded in front of him. "I saw the trees falling and the glass shattering in front of me. I had to run away from the car," he said.
Zerhouni said the attack was the work of the Salafist Group for Preaching and Combat (GSPC), the same group that took responsibility for an attack in April in downtown Algiers that killed 33 people.
That group also uses the name al Qaeda Islamic Maghreb after merging with al Qaeda earlier this year. It abandoned small-scale attacks in favor of headline-grabbing blasts after it joined with al Qaeda.
CNN International Security Correspondent Paula Newton said the merger combined the expertise of Algerian guerrillas with the operational ability of al Qaeda in North Africa, enabling the group to penetrate the usually extensive security in high-profile areas of Algiers.
She said the group's goal is to destabilize countries like Algeria, Morocco and Tunisia, which it sees as enemies of the Islamic state.
Zerhouni said police interrogations of GSPC members arrested in the wake of the April attack revealed that Algeria's Constitutional Council and Supreme Court were on a list of GSPC targets.
Algeria, which has a population of 33 million, is still recovering from more than a decade of violence that began after the military government called a halt to elections which an Islamist party was poised to win.
Tens of thousands of people died in the unrest. Although the country has remained relatively peaceful, recent terrorist attacks have raised fears of a slide back to violence. E-mail to a friend