The UN has confirmed 10 of its staff died in a double car bombing in the Algerian capital, Algiers, which killed at least 26 people and injured 177.
Medics believe up to 62 people died when the devices went off, shearing the facade off the UN refugee agency and hitting a government building.
Secretary General Ban Ki-moon called the worst attack on the UN since 2003 "an abjectly, cowardly strike".
Al-Qaeda's North Africa wing claimed responsibility on an Islamist website.
Algerian television has reported that six people were pulled alive from the ruins of the UN offices.
"We now believe that the UN death toll is at 10," said UN spokeswoman Marie Okabe, adding that "a number of staff" were still missing.
The neighbouring office of the UN Development Programme was also damaged when the first car bomb went off at around 0930 (0830 GMT) in the Hydra district.
It was the deadliest attack on the UN since militants bombed its offices in Baghdad, Iraq, in 2003, killing 22 people.
At around the same time, an explosion ripped into a bus packed with students outside Algeria's Constitutional Council in the Ben Aknoun district.
The body rules on the constitutionality of laws and supervises elections.
In an unverified online statement, a group calling itself al-Qaeda in the Land of the Islamic Maghreb said two vehicles packed with 800kg (1,700lb) of explosives each were used in the blasts.
The group labelled the UN buildings an "international infidels' den" and said it had carried out the attacks to honour one of its senior militants, who died fighting Algerian troops.
It called on Western leaders to "heed the demands of al-Qaeda leader Osama Bin Laden".
UN High Commissioner for Refugees (UNHCR) Antonio Guterres said his office was "of course a target for those that have a completely nihilist vision of today's world".
Algerian Interior Minister Yazid Zerhouni confirmed a suicide bomber was behind the blast which hit the UN offices.
The UN secretary general led international condemnation of the attacks.
ATTACKS IN ALGERIA IN 2007
11 December: twin car bombs kill at least 26 including 10 UN staff in Algiers
8 September: 32 die in bombing in Dellys claimed by al-Qaeda in the Islamic Maghreb
6 September: 22 die in bombing in Batna claimed by al-Qaeda in Islamic Maghreb
July: Suicide bomber targets barracks near Bouira, killing nine
May: Dozens killed in run-up to elections, in fighting between military and militants
April: 33 killed in Algiers in attacks claimed by al-Qaeda in the Islamic Maghreb
March: Three Algerians and a Russian killed in attack on gas pipeline workers
February: Seven bombs kill six east of Algiers
"This was an abjectly cowardly strike against civilian officials serving humanity's highest ideals," he said.
US President George W Bush condemned the attack "by these enemies of humanity" and French President Nicolas Sarkozy called the bombings "barbaric and cowardly".
Algerian Prime Minister Abdelaziz Belkhadem, who visited the injured in hospitals, said nothing could "justify the crime".
There have been a series of bomb attacks across Algeria during the past year in which scores of people have died.
Members of the public have recently held rallies in protest at the upsurge in violence.
Many of the recent blasts were also claimed by al-Qaeda in the Land of the Islamic Maghreb (AQLIM).
Algeria suffered a brutal and bloody civil war in the 1990s, but in recent years violence had declined.