UN staff returning to Iraq will have to rely on the US-led multinational force for protection, UN Secretary-General Kofi Annan says.
Danish troops are part of the 32-nation force
The UN pulled out all its staff in October after two suicide bomb attacks on its Baghdad headquarters.
Mr Annan said the UN had not received any firm offers from countries for the separate dedicated protection force it had hoped to have in place in Iraq.
A small UN team is due to return to Iraq later this month.
Mr Annan told UN Security Council ambassadors on Wednesday that at least 5,000 troops would be needed to protect the much larger UN contingent needed to assist with elections and reconstruction projects.
The small UN team led by veteran Pakistani diplomat Ashraf Jehangir Qazi is expected to go to Baghdad shortly to take part in a national political conference.
"We are forced to rely on the multinational force to give us protection," Mr Annan said.
"We haven't had much success attracting governments to sign up for the dedicated force."
No deals with kidnappers
Meanwhile, the US said all 32 countries in the multinational force in Iraq had agreed not to make concessions to hostage-takers.
"We are united in our resolve to make no concessions to terrorists," the State Department announced.
This is a clear message to militants that seizing foreigners to put pressure on governments to withdraw their troops will not work, correspondents say.
Six Jordanian and Turkish hostages were released earlier on Wednesday.
About 20 other foreign nationals are still being held captive in Iraq.
There has been a dramatic increase in kidnapping in recent weeks as militants try to exert emotional pressure on foreign governments to pull out, and on overseas companies to stop doing business in Iraq.
While several private firms have agreed deals to secure the release of their employees, the US said countries in the multinational force would not yield.
State Department spokesman Richard Boucher said the idea for a common policy statement on hostage-taking was first proposed by Bulgaria.
Four Jordanian men were freed but the fate of three others is unclear
The US had then followed it up with each of the other governments in the multinational force and reached an agreed principle to be expressed by individual nations in their own words.
The co-ordinated campaign aims to signal to militant groups that there would be no "weakening of resolve", the US statement read by Mr Boucher said.
Countries with troops in Iraq or whose nationals have been working for companies supplying the US military have been particular targets.
As well as sending a clear message to militants, it is also a response to the Philippines, which withdrew its own troops early to ensure the safe release of one of its nationals, says the BBC's Jill McGivering in Washington.
That move angered the US and increased pressure on other members of the multinational force facing similar demands, she reports.
On Wednesday four Jordanians - three of them lorry drivers - were freed in Falluja, apparently after a group of Iraqis stormed the place where they were being held and set them free.
Two Turkish drivers, Abdulrrahman Demir and Said Unurlu , were also released by militants from Tawhid and Jihad, who said their haulage firm had agreed to stop work in Iraq.
Tawhid and Jihad earlier announced it had shot dead another Turkish lorry driver, Murat Yuce.
The four Jordanians have been named as Ahmad Abu Jaafar, Mohammad Ahmad Khleifat, Khaled Ibrahim Masoud and businessman Ahmad Tayseer Sunokrot.
One of the hostages told Reuters news agency by telephone that local people had raided the house where they were being held.
Their captors had called on the people of Jordan to force their government to end support for the new government in Baghdad.
A company which employs two of the men also pledged to halt operations in Iraq - another demand of the kidnappers.