UNITED NATIONS - Five U.N. staff were still missing after a bombing in Algeria in which 11 other U.N. employees died, the world body said on Thursday.

Al Qaeda's North African wing claimed responsibility for twin car bombs on Tuesday that killed more than 30 people in all at the U.N. offices and a court building in Algiers, saying it had targeted what it called "the slaves of America and France."

U.N. spokeswoman Marie Okabe said the confirmed U.N. death toll so far was 11, and five others were missing.

"The number of casualties could rise as efforts to recover bodies continue," she said in New York.

U.N. Development Program Administrator Kemal Dervis was in Algiers on Thursday to meet the wounded and the families of the dead and to discuss security with the government.

Asked whether Secretary-General Ban Ki-moon would be visiting Algeria himself, Okabe said the United Nations had lost its operational capacity there as a result of the bomb.

"A visit under these circumstances, at this time, would be extremely difficult," she said, adding that Ban had decided to stay longer than planned in Bali for a climate change conference where he wants to push ahead tough negotiations.

Ban promised on Wednesday to review security at U.N. sites around the world and Okabe said that process had already started in Algeria.

At U.N. headquarters on the east side of Manhattan, New York city police deployed more forces than usual on Thursday on the street in front of the building, including a surveillance tower and more armed guards.

Okabe declined to comment on whether any specific threat had led to the heightened security.

"Security (department) is constantly monitoring all information and it takes appropriate measures," she said.