The UN Security Council has unanimously approved a resolution calling for a greater UN mandate in Iraq.
The UN withdrew most of its staff in 2003 after a bomb attack on its Baghdad headquarters killed its top envoy, Sergio Vieira de Mello, and 21 others.
But the UN Staff Council has called on Secretary-General Ban Ki-moon to pull all UN personnel out of the country until security improves.
The union can resist a further deployment of staff in Iraq.
The staff union believes UN personnel will not be properly protected by US-led forces in the country.
The resolution extends the mandate of the UN mission in Iraq for a year and gives the UN a more powerful advisory role in Iraq.
It will pave the way for the UN special envoy in Iraq to support and assist the Iraqi government in political, economic, electoral, and constitutional matters, and help settle disputed internal boundaries
The UN mission would also be asked to promote human rights and judicial and legal reforms and to assist the Iraqi government in planning for a national census.
The number of UN staff in Iraq would be raised from 65 to 95.
But correspondents say there will be limits as to how much the UN can do on the ground.
The US ambassador to the UN, Zalmay Khalilzad, has said that the UN could play a positive role in facilitating dialogue between rival factions in Iraq, especially those that refuse to talk directly to the US.
"This is an effort to internationalise the effort to assist Iraqis overcome their internal difference, and to assist the neighbours by bringing them together to help Iraq rather than add to Iraq's problems," Mr Khalilzad told the Associated Press news agency.
Some observers have said that the expanded UN mission is a way for the Bush administration to get the international community to pick up the pieces in Iraq after years of chaos.
But supporters of the move, including at the UN, believe this may promote progress in Iraq and help break the political deadlock there.