The advance team arrived on Friday as prominent Iraqi clerics remained sharply divided over the UN's involvement in Iraq's reconstruction and upcoming elections.
"The opening of a direct line of communication with the US-led coalition on security matters is necessary for the planning for the safety and security of UN personnel, activities and assets in Iraq and for an eventual return of UN international staff to Iraq," UN spokesman Stephane Dujarric said.
He said the team that reached Baghdad consists of a military advisor and a security coordinator.
"This team's primary purpose will be to liaise with the Coalition Provisional Authority and the coalition forces, which is very important in achieving enhanced safety and security for our national United Nations personnel as well as that of UN premises," Dujarric said.
UN pulled out its international staff from troubled Iraq after a deadly bomb explosion in August last year at its Baghdad headquarters, which left more than 20 people dead.
It has been under pressure from the US to return since then.
But fractious ground realities in Iraq suggested that the road to the UN's possible return was lined with pitfalls.
Two prominent Iraqi Shia clerics have during the day differed publicly over the UN's "utility."
Leading cleric Ali al-Sistani urged a halt to mass protests against US plans for handing over power until the UN had studied the feasibility of holding direct elections.
Cleric Ali al-Sistani urged a halt to mass protests against US plans for handing over power till the UN had studied prospects of holding direct elections
But the more radical cleric Moqtada Sadr branded the United Nations a "dishonest" body that served America's agenda and had no role to play in the future of Iraqi elections.
The differences underscored once again the difficulties confronting restive Iraq.
Huge Shia protests in recent days against US plans to transfer power to an unelected provisional government have prompted Washington to seek a compromise.
US-occupation administrator Paul Bremer and members of the Iraqi Governing Council on Monday held talks with UN chief Kofi Annan, calling for the world body's return.