Shirin Tahir-Kheli, a top US State Department adviser on UN reform, told a General Assembly debate that a proposal from Brazil, Germany, India, and Japan lacks support and a vote now would be too divisive.
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The US Senate would determine whether any expansion "makes the council more or less effective" and the Bush administration wanted no change, she said.
"Let me be as clear as possible: The US does not think any proposal to expand the Security Council -- including one based on our own ideas -- should be voted upon at this stage," she said.
Her statement underscored just how divided member states remain on UN Security Council reform despite weeks of debate. It also comprised the most detailed and forceful US statement in the latest round of debate on Security Council reform, an issue that has bedeviled the UN member states for over a decade.
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Russia, China and now the US oppose the resolution, which calls for expansion of the council from 15 to 25 members, adding six permanent seats without veto power and four non-permanent seats. Those four each want a permanent seat, with the other two earmarked for Africa.
Britain, France and most Europeans support the proposal, though Britain seems to be wary of formal support given the US opposition.
Opposition also comes from the Uniting for Consensus group, which includes Pakistan, which rejects India's candidature, Spain and Italy, (which oppose Germany's bid) and Latin American countries like Mexico and Argentina which resent Brazil's aspirations. China has made clear it would oppose any bid by Japan for a permanent seat.
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Germany's UN Ambassador Gunter Pleuger said he expected Germany and its allies on council reform would seek a vote on their proposal next week nonetheless. Any proposal would need two-thirds approval from the 191-member General Assembly.
``I think the vote will come,'' Pleuger said after the debate ended. ``We feel that the votes are there.''
In March, UN Secretary-General Kofi Annan said he wanted a decision on council expansion before September, when world leaders will gather at the United Nations for a summit.
Yet the prospects for a resolution now have clouded. Last week, the 53-nation African Union came forward with its own council reform idea. A third group, called Uniting for Consensus, doesn't want to create any new permanent members and has remained steadfast in its opposition.
The United States had said weeks ago that it wanted progress on other reform proposals before going ahead with an overhaul of the Security Council. It also favors adding just one or two new permanent seats and expanding the council to 19 or 20 members.
``We will work with you to achieve enlargement of the Security Council, but only in the right way and at the right time,'' Tahir-Kheli said.
Earlier Tuesday, Annan reiterated his concern that council reform must not overwhelm his other reform proposals, known within the United Nations as clusters.
``There's no reason, no excuse not to bring those discussions to closure and if they have to focus on the Security Council reform, let them do that, but they should move on the other clusters as well,'' he told reporters.
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