By Laura Trevelyan
BBC News, New York
The two bombings in Algiers on Tuesday by an affiliate of al-Qaeda killed at least 11 UN employees. It appears UN offices were deliberately targeted.
UN staff are worried at the way their colleagues were targeted
The attack was the deadliest against UN staff since August 2003, when 22 were killed by a bomb in Baghdad.
At the world body's headquarters in New York, the UN flag is flying at half mast, a mark of respect to those who died in the line of duty in Algeria.
The mood at the UN is sombre and staff have called for a full investigation.
They want to discover whether adequate security measures were in place to prevent such a horrifying act.
'Attack on us all'
Babacar Ndiaye, Hanniche Abel-Rahim, Nabil Slimani, Adnane Souliah, Kamel Sait, Hind Boukroufa, Djamel Rezzoug, Steven Olejas and Gina Luna were among those killed.
Diplomats in the General Assembly hall on Wednesday applauded as the Secretary General, Ban Ki-moon, said via video link from Bali: "Let us know that this attack on the UN is an attack on us all and our highest ideals."
For UN staff in New York, this is the worst attack on their colleagues since the 2003 bombing of the UN's headquarters in Iraq. Among those killed was the revered head of the mission, Sergio Vieira de Mello.
"It claimed the lives of some of our best and brightest," said Mr Ban.
"We have taken many measures since then to enhance the security of our staff and premises around the world."
What is so worrying for UN staff is the way the organisation itself seems to have become a target for militants. Diplomats describe this as an alarming development.
UN employees see themselves as neutral arbiters in the world's many complex disputes - there to help, not be attacked.
As the UN High Commissioner for Refugees, Antonio Guterres, said: "The UN is an entity that works for world peace, an entity that tries to be an honest broker, especially when dealing with the humanitarian needs of refugees and other victims of violence and persecution.
"This makes this type of attack even more absurd," he added.
In the staff canteen that concern was echoed.
"The targeting of UN workers is now very new and unfortunate," Natabara Rollosson from the UN children's agency, Unicef, told the BBC.
"In the past, it's my understanding, it has been an unspoken rule that they are just off limits as targets," he added.
"It's like the people who carry stretchers in war, there's no use and there's no purpose to target them. But for some people they see humanitarian work as a political component of war. And I think that shift is really unfortunate."
A man who wished to remain anonymous said: "The missions are still worth it because the ideals and the goals are still much higher and more honourable."
"And I think the people who died, died for a worthy cause. As civilians, it's very sad, but I guess it's the price we pay for fighting for peace and fighting for stability."
Before the Algiers attack, 19 UN staff had been killed during 2007.
The UN has a duty to protect its staff, but it is also trying to help people, and so it must be accessible. That creates a vulnerability which can be exploited.