The African Union (AU) has stuck to its demands for two permanent seats with veto powers in an expanded UN Security Council, despite pressure to back down.
Kofi Annan wants a decision on any changes by September
Members rejected a plea to drop their veto request at an extraordinary summit in Ethiopia's capital, Addis Ababa.
It will be a blow to the so-called G4 - Brazil, Germany, India and Japan - who wanted a changed AU stance to boost their own hopes for extra seats.
UN chief Kofi Annan wants members to decide on an expanded council soon.
Chief AU spokesman Desmond Orjiako told reporters Africa's heads of state had "endorsed" the AU position taken on UN expansion at its annual summit in Libya.
They have "set a committee of 10 to propagate the African position to the international community," he added.
Some 46 of the AU's 53 members backed the position following a contentious debate of the issue, the AFP news agency reported.
At the meeting in Libya, the AU called for the 15-member Security Council to be expanded to 26.
It wants six new permanent seats with veto-wielding powers - of which two would be for Africa - and five new non-permanent seats, two of which would also go to Africa.
Since then, the AU has met the G4, which envisages expanding the council to 25, with six new permanent members - four for them, and two for Africa - without veto-wielding powers.
The argument for not having a veto - a power currently wielded by the present permanent members, China, France, Russia, the UK and the US - is that it would only paralyse the UN still more, preventing it from acting in a crisis.
The AU has not yet decided on the deeply divisive issue of which African countries would take the two seats on an expanded Security Council.
Egypt, Nigeria and South Africa are the leading candidates for the seats, being regional powerhouses.
But a number of other states - including Angola, Kenya and Algeria - are not happy about being relegated to the second division, says the BBC's Martin Plaut.