United Nations officials have underlined their intention to play no role in Iraq's political future unless the US revises its plans for the country.
Mr Powell says he would welcome suggestions from critics
A senior UN official told journalists that Washington would have to undertake a radical revision of the draft resolution on Iraq submitted to the Security Council.
The remarks echo unusual criticism expressed by the UN Secretary-General, Kofi Annan.
France, Russia and Germany have also expressed their concern, with Paris calling for a stronger role for the UN in Iraq.
US Secretary of State Colin Powell said America was looking for "specific suggestions" on improving the resolution, ahead of Monday's meeting of the Security Council.
Mr Annan made it clear at a private lunch with the 15 Security Council ambassadors that the UN was not prepared to risk its staff for the limited political role proposed by the US, a senior UN official said on Friday.
The BBC's UN correspondent Greg Barrow says it is rare for the UN to voice such candid opposition to plans put forward by one of its member states - not least the most powerful.
US DRAFT'S KEY POINTS
Bigger UN role
Progressive handover of power to Iraqis
New constitution and elections
Multi-national force under unified command
Effective Iraqi police and security forces
Full range of loans for reconstruction
But such is the level of concern about the situation in Iraq that Mr Annan and his officials appear to have decided that the time has come to speak plainly.
Mr Annan believes the US proposals would expose UN staff to added risk after two bombings at the UN's headquarters in Baghdad which killed 23 people, the unnamed UN official said.
Responding to the criticism, US Secretary of State Colin Powell telephoned Mr Annan to assure him the US wanted a stronger role for the UN in Iraq, a US official said.
But analysts say the US is looking increasingly isolated, with Mr Annan's comments exacerbating the doubts which many Security Council members already have.
Our correspondent says diplomats at the UN are already warning that, if Washington is pushed too hard, it may decide to walk away from the UN.
Security in Iraq is a problem despite the US presence
The draft resolution - due to be discussed by council members next week - calls for the political transition to be as quick as possible and urges member states to contribute to a multinational force.
BBC diplomatic correspondent Barnaby Mason says the draft does not specify how the UN's role would be strengthened, and the implication is that the US-led coalition will continue to have the predominant voice.
A major sticking point is the chronology of a return to democracy.
The US wants to establish a new Iraqi constitution first and then hold elections. The UN believes more can be achieved by establishing a transitional government, restoring democracy and then elections.
France said the draft failed to incorporate changes to an earlier version that both France and Germany had asked for.
"Our first impression is that our concerns are not reflected in the revised US draft resolution except in a very limited way," said foreign ministry spokesman Herve Ladsous.
Russian President Vladimir Putin was also critical, saying said Moscow was "not satisfied" with the latest draft .
The US has said it wants to get a resolution passed before an international conference to raise money to help rebuild Iraq later this month.
France has said that, despite its misgivings, it will not veto a resolution when it is put to a vote.