UN Secretary General Ban Ki-Moon has ordered a security review after a bomb in Algiers which was the deadliest attack on its offices since 2003.
At least 11 UN employees died when a car bomb blasted the facade of the UN refugee agency in the city on Tuesday.
A second bomb shattered an Algerian government office, reportedly killing and injuring students in a passing bus.
Al-Qaeda's North Africa wing claimed both attacks, which killed at least 26 people and injured nearly 180.
"We will take every possible measure to aid those injured in the attack and their families," Mr Ban said, speaking in Bali where he is attending the UN climate change conference.
"The safety and security of the United Nations staff is paramount.
"We will take every measure to ensure this, in Algiers and elsewhere, beginning with an immediate review [of] our security precautions and policies."
Mr Ban called the attack "an abjectly, cowardly strike". It was the most lethal bomb attack on a UN office since the Baghdad bombing of 2003.
Security has been high at the Bali talks but a UN security officer told AFP news agency it had "gone up another notch after what happened in Algiers".
"If these people will hit a refugee office, they'll attack anything," the unnamed officer added.
Pulled out alive
The Algerian government's official death toll for the twin attacks is still 26 with 177 people injured.
But medics believe up to 62 people died.
Six people were pulled alive from the ruins of the UN offices.
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.
At around the same time, an explosion ripped into the 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.
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
The group labelled the UN buildings an "international infidels' den" and called on Western leaders to "heed the demands of al-Qaeda leader Osama Bin Laden".
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.
In September more than 50 people were killed in suicide attacks - one of them involved a truck packed with explosives being driven into a coast-guard base.
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), including a triple suicide bombing in Algiers in April which killed 33 people.
Algeria suffered a brutal and bloody civil war in the 1990s, but in recent years violence had declined.