The UN Security Council has unanimously approved a US-British resolution calling for a greater UN role in Iraq.
The UN staff union is against plans to send more staff to Baghdad
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 there is opposition to a wider role from the UN Staff Council, which wants all UN personnel to be pulled out of the country until security improves.
The staff union can resist a further deployment of staff in Iraq.
It believes UN personnel will not be properly protected by US-led forces in the country.
UN Secretary-General Ban Ki-moon Ban welcomed the result of the vote, saying the international community would support Iraqi efforts to create "a peaceful and prosperous future" for themselves.
"The United Nations looks forward to working in close partnership with the leaders and people of Iraq to explore how we can further our assistance under the terms of this resolution," he said.
The resolution - approved by all 15 Security Council members - extends the mandate of the UN Assistance Mission in Iraq (Unami) 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.
Authorises UN envoy to help Iraqi government in political, economic, electoral, constitutional, refugee and human-rights issues
Promotes talks among ethnic and religious groups
Promotes talks between Iraq and neighbours on border security, energy and refugees under UN auspices
Raises UN staff in Iraq to 95
"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.
This resolution underscores the shifting approach of the Bush administration to Iraq, says the BBC's diplomatic correspondent Jonathan Marcus.
The US is desperate to reduce its military entanglement in Iraq, believing regional countries have a role to play in reducing violence there, our correspondent says.
Residents of the Iraqi capital, Baghdad, told the BBC's Arabic Service they thought the UN deployment would not influence the sectarian rivalries threatening stability in the country.
"All [the UN's] resolutions and its presence are not worth the paper they are written on... Change must be radical, bringing in a secular government, a non-factional government that does not side with the Shia or align itself with the Sunnis," one man said.
But supporters of the move, including at the UN, believe this may promote progress in Iraq and help break the political deadlock there, analysts say.