Kofi Annan said on Friday the US will have to give some ground if a new security council resolution urging US allies to send troops to Iraq is to be passed.
He also insisted there is no question of sending an official UN peacekeeping force to the war-torn country.
"But it is not excluded that the Security Council may decide to transform the operation and to have a UN-mandated multinational force operating on the ground," he said.
"But of course ... it would also imply not just burden-sharing but also sharing decisions and responsibility. If that doesn't happen, I think it's going to be very difficult to get a second resolution."
The UN Secretary-General named Ramiro Lopes da Silva, the UN humanitarian coordinator for Iraq, to be his interim special representative to Iraq, diplomats said on Friday.
He succeeds Sergio Vieira de Mello, who was killed in Tuesday's bombing of the UN headquarters in Baghdad.
But the United States has indicated it is determined to press for a new resolution even at the risk of reopening old wounds with its allies over Iraq.
US Secretary of State Colin Powell said on Thursday the United States is exploring a new resolution that might spur the deployment of more international troops in Iraq.
But he also made it clear there is no question of giving up US operational authority, which many countries have made a condition for sending troops.
The Security Council has yet to heal bitter divisions over the initial military invasion, and Annan has advised all discussions on a new resolution be held behind closed doors.
However, the United States is convinced the bombing of the UN headquarters in Baghdad this week has altered the climate for debate over the conflict.
"We think we have an environment to do this now and we think that anyone that would want to stand in the way of the reconstruction of Iraq and stand in the way of supporting the Iraqi people and the United Nations should be held to account," a senior State Department official said.
Abd Alllah Gul says parliament will
decide if Turkish trrops go to Iraq
"This is not the time for a sterile debate on rights and responsibilities but a time to stand up for the international community," he added.
Meanwhile, Turkey’s foreign minister has said Turkish troops will command their own sector of operations if they are deployed to help US forces in Iraq.
Foreign Minister Abd Allah Gul told the liberal daily Milliyet that Ankara and Washington had agreed any Turkish troops would control their own separate region to the north or west of Baghdad.
"When the US made its first proposal at military headquarters it said 'You will be free'," Gul said.
"There will be a special sector under Turkish command and with a separate chain of command. We will decide where we will serve."
Turkey is working to repair relations with the United States after refusing America permission to launch an invasion of Iraq from its soil.
But Turkish public opinion is against sending troops to join the US-led force and analysts say the issue could splinter the ruling Justice and Development Party.
Parliament's speaker and some deputies say Turkey should not go in without UN backing.
Gul said the government had not yet made any firm decision on sending troops, and parliament will have the final say.