Last-minute negotiations have begun at the United Nations headquarters in New York to try to reach agreement on ways of reforming the organisation.
UN Secretary General Kofi Annan has put forward plans for wide reforms
The General Assembly had produced a draft document which it hoped would be adopted at next month's world summit to mark the 60th anniversary of the UN.
But last week, the US produced a series of objections to the document.
Critics fear the UN could end up with a short statement of principles rather than one of substantive reforms.
Negotiations carried on into the night on Monday, and will resume on Tuesday. The BBC's Michael Voss in New York says it is a race against time.
A group of around 30 countries has been given the task of reaching a consensus on a range of contentious issues before world leaders gather in New York in mid-September.
The UN Secretary General, Kofi Annan, had hoped to use the summit to push through a series of reforms aimed at redefining the organisation for the 21st Century.
These include management changes as well as fresh action to meet its millennium goals to reduce poverty and promote development.
After six months of discussions, a 38-page draft document was presented to the General Assembly earlier this month.
Then, last week, the new US Ambassador to the UN, John Bolton, stepped in, asking for hundreds of changes.
The US objects to references to the International Criminal Court, action on global warming and increases in development aid.
At the same time it wants greater commitment to tackling terrorism and nuclear proliferation.
The UN has set itself a deadline of the end of this week to produce a new document.