Four countries led by Brazil have formally introduced a proposal to enlarge the UN Security Council.
Mr Annan has backed expansion
Brazil, Germany, Japan and India want to add 10 more seats to the 15-member Council, six of them permanent. These would go to the four states and Africa.
Brazil's ambassador to the organisation said the realities of power had changed since 1945, when the UN was founded.
Correspondents say there is widespread support for additional members, but deep division over how to achieve this.
The proposed expansion is part of a broader plan to overhaul the UN, which is spearheaded by Secretary General Kofi Annan.
The plan's advocates say a vote could take place as early as this week.
The so-called Group of Four (G4) put forward the proposal in the UN General Assembly, where it would need a two-thirds majority - and then ratification by all existing permanent security council members - to come into effect.
At present, the US, the UK, France, Russia and China are the only permanent members of the UN body, with the power to veto. Ten other nations rotate in two-year terms.
Brazilian ambassador Ronaldo Mota Sardenberg said the Council needed to become more balanced.
But critics argued that the G4 plan was a bid for power.
"The seekers of special privileges and power masquerade as the champions of the weak and disadvantaged," Pakistani UN ambassador Munir Akram said.
Pakistan backs a different plan from a group known as Uniting for Consensus, which proposes adding 10 new non-permanent members who would face re-election.
For its part, the African Union would like to see six new permanent seats - but the organisation wants them to have the veto, whereas the G4 has dropped this demand.
Washington is calling for two new permanent seats with no veto power, including one for Japan.
China has called for further consultations on the issue.
Ambassador Wang Guangya said Beijing "rejects the forcible vote on any formula on which there still exists significant differences."
But France is among more than 20 countries who have sponsored the G4 plan.
The BBC's Susannah Price at the UN says the debate has overshadowed other discussions on reform.
Mr Annan says he would like a decision on Council expansion before the UN summit of world leaders in September.
In March, he set out proposals for a number of sweeping reforms to the organisation, which he said were needed to meet "today's realities".
Other ideas include an agreed definition of terrorism, new guidelines for authorising military action and the streamlining of the General Assembly agenda.
The Assembly comprises all 191 UN member states.