"My impression is that they are working for a multinational force, under a unified command, which would cooperate with the sovereign government of Iraq and contribute to the security of the country," Annan told Italy's Corriere della Sera newspaper on Monday.
The UN has previously demurred on any deployment of its blue-helmet troops to Iraq, where US-led forces, without UN backing and in the face of opposition from Security Council members, last year led an invasion to overthrow Saddam Hussein.
Despite differences over the justifications for going to war, the US administration has in recent months urged the UN to take a more active role in Iraq, where US troops still face daily insurgent attacks a year after Saddam's fall.
More than 20 people were killed
in a bomb attack at the UN HQ
A UN team of electoral experts arrived in Baghdad on Sunday to help plan for elections due early next year.
A second team, led by senior UN envoy Lakhdar Brahimi, is due to arrive in the coming week to help decide on the shape and structure of an Iraqi government to take power from 1 July, after US authorities hand sovereignty back to Iraqis.
From that date, Iraq will theoretically no longer be under US-led occupation, opening the way for countries and organisations other than those in the US-led coalition to be invited to participate in helping rebuild and secure Iraq.
Spain has been one of the staunchest allies in the US coalition with more than two thousand troops deployed in Iraq. But the country's incoming prime minister has pledged to withdraw those forces from June 30 unless the UN is involved.
As well as the possibility of a troop deployment, the UN is also making plans to return international staff to Iraq. Staff were withdrawn in October last year after two deadly bombings against UN buildings which killed more than 20.