The United Nations High Commissioner for Human Rights has been appointed special representative for Iraq.
UN Secretary General Kofi Annan named Brazilian diplomat Sergio Vieira de Mello to the post a day after the Security Council approved a resolution giving its backing to the US-led administration.
Mr Vieira de Mello, 55, will retain his human rights post, which he took up only last year.
The Security Council resolution calls for the United Nations to take a role in post-war Iraq, but the BBC's Jon Leyne in Washington says it will likely be a limited one.
Fourteen of the 15 Security Council members voted to adopt the resolution, which lifted sanctions on Iraq.
France, Russia and Germany - all countries which opposed the war - backed the resolution, while the sole Arab state on the council, Syria, did not attend the meeting.
Voting took place nine weeks to the day after US-led coalition forces invaded Iraq at the start of a military campaign that toppled Saddam Hussein's regime.
The changes to earlier drafts included an expansion of the role of the UN - a key demand of Russia and France, two countries with the power of veto.
RESOLUTION KEY POINTS
US and UK to maintain most of the power
Occupying forces to remain till new government formed
Situation to be reviewed within a year
UN to appoint special representative to help form new government
Sanctions to be lifted, though arms embargo will stay
Russian and French companies will be able to complete lucrative contracts
Return of UN weapons inspectors to be considered
$1bn Iraq Development Fund to be launched
The occupying powers, the US and Britain, are left firmly in control of Iraq and its oil "until an internationally recognised, representative government is established".
The resolution sets a legal framework for the future of Iraq, and will give the go-ahead for companies wishing to invest in the country.
"The lifting of sanctions marks a momentous event for the people of Iraq. It is time for the Iraqi people to benefit from their natural resources," US Ambassador to the UN John Negroponte said after the vote.
French ambassador Jean-Marc de la Sabliere said the resolution was not perfect, but added: "The Iraqi people must now take control of their future."