UN Secretary General Kofi Annan has chosen a new chief of staff to help reform the world body, as it deals with corruption claims and strained US ties.
Malloch Brown (left) will try to reform the UN in a time of crisis
The new chief, Mark Malloch Brown, will keep his present job as head of the UN Development Programme.
The 51-year-old Briton is replacing Mr Annan's long-time confidante, Iqbal Riza, whose resignation was announced last month amid UN staff unrest.
Mr Malloch Brown acknowledged the UN was facing a difficult period.
The appointment is seen as an attempt to steady the UN ship, the BBC's diplomatic correspondent Jonathan Marcus reports.
It comes as UN staff morale continues to tumble and amid lingering allegations of corruption in the UN-administered programme to purchase Iraqi oil in return for food - now the subject of a number of investigations, our correspondent says.
Last month, Mr Annan met a group of veteran US foreign policy experts - who are not members of the government - to discuss actions necessary to deal with the UN's problems, the New York Times newspaper reports.
Mr Malloch Brown is widely credited with reforming the UN Development Programme, which operates in more than 160 countries around the world.
There are several inquiries into the Iraq oil-for-food programme
He took office in 1999 after moving from the World Bank where he was a vice-president in charge of external affairs and public relations.
His appointment by Mr Annan is only the first step in what promises to be a much wider shake-up of the UN's top jobs, our correspondent says.
There has been much criticism of the way UN managers behave, while neo-conservative voices in Washington have attacked Mr Annan himself, urging he be replaced.
A few weeks ago, Mr Annan received a report from a high-level panel that he established, setting out a number of possible reforms.
It is clear the catastrophe in south-east Asia is also putting the organisation's machinery to the test, our correspondent says.
But it may also provide an opportunity - a terrible and all too graphic demonstration that some problems are just too big for individual countries or even groups of countries to handle, and must be dealt with at the global level, he adds.