The organisation has not claimed responsibility for the attacks.
Zerhouni had said hours earlier that vehicles packed with explosives were the method for the bombings, and that one bombing was a suicide attack.
"For the attack which took place outside [the UN buildings] it seems that it was carried out by a suicide bomber," he said.
At least four UN staff members were killed in the attack on the UN buildings, a UN spokeswoman said.
She said that 14 members of staff were still unaccounted for.
Those killed were working in the offices of the UN High Commissioner for Refugees (UNHCR) and UN Development Programme (UNDP), the official said.
"They are trying to locate people in hospitals, they are digging through the rubble," Maria Okabe, a UN spokeswoman in New York, said.
The al-Qaeda in the Islamic Maghreb organisation has claimed responsibility for a number of bombings that have killed several people this year.
"[The attacks] come after a speech by Ayman al-Zawahiri, al-Qaeda's deputy leader, this year that he had asked attackers to target France, Spain and Algeria," Hashem Ahelbarra, Al Jazeera correspondent, said.
|The bombings come weeks after attacks |
by an al-Qaeda inspired group [AFP]
In September, a car bomb killed 37 people at a coastguard barracks in Dellys, 100km east of Algiers, two days after a suicide bomb blast targeting a convoy carrying Abdulaziz Bouteflika, Algeria's president, killed 22.
Both of the attacks were claimed by the group, which was formed from the Salafist Group for Preaching and Combat (GSPC).
The GSPC was formed in 1998 by former members of Algeria's Armed Islamic Group (GIA), which began attacking the government in 1992 after it cancelled elections an Islamic party looked set to win.
More than 150,000 people died in an ensuing decade-long civil war.
Al-Qaeda in the Islamic Maghreb was formed from the GSPC in January 2007, with leader Abu Musab Abdel Wadoud promising to wage a violent campaign.
The US condemned Tuesday's attack on the UN buildings, with the White House saying it would support Algeria and the UN in the wake of the attacks.
"We condemn this attack on the United Nations office by these enemies of humanity who attack the innocent," Gordon Johndroe, White House spokesman, said.
Nicolas Sarkozy, France's president, condemned the attacks in the former colony as "barbaric", in a telephone call to his Algerian counterpart.
"President Sarkozy has just called president Bouteflika to express the French people's solidarity and compassion towards the Algerian people," David Martinon, presidential spokesman, said.