In high-level talks with Pentagon officials at UN headquarters, officials warned that President George Bush's original plan to have the operation completed by 20 January - inauguration day - could sow the seeds of disaster.
The US side stunned UN officials by declaring there were no plans to disarm the warring clans in Somalia and that troops would only be sent to the most troublesome spots: Mogadishu, Bardera and Baidoa. When a UN official said there was also trouble in Hargeisa in the north, the US officials had to consult a map to see where it was, according to sources present at the meeting.
Another possible flashpoint between the United States and the UN concerns the operation's cost; the UN has told Washington it cannot set the cost of the mission against its dollars 700m (pounds 460m) arrears at the organisation. The US is expected to seek large contributions from Britain, Japan, Germany and other countries not sending forces to Somalia.
'It's the aftermath of the US operation that's most worrying,' said a diplomat whose government is debating whether to join the US operation or the separate UN peace-keeping mission, which will be in Somalia long after the marines have left. 'The US does not see its mandate as stabilising the country by removing arms, and since the UN is not paying for the operation or running it in any meaningful way there is little it can do about it.'
The US has made clear it will not tolerate micro-management of the operation by officials in New York. The UN, meanwhile, has been manoeuvring to find a way to keep the US engaged in Somalia for as long as possible. Officials say deployment of the peace-keeping force that is to take over from the US could be delayed to ensure the US stays put well into Bill Clinton's presidency.
The UN fears the US plan to send forces to Mogadishu and to a few areas where aid agencies have set up food distribution centres is not enough. The fear is that Somali fighters will hide their weapons when the US troops arrive, only to begin fighting again once the marines have left.
The US-led force will operate under a unified command, as in the Gulf war, but with much more stringent reporting requirements to the Security Council. The US troops will not be considered UN forces, answerable to the Secretary-General, but the existing UN peace-keepers will remain under UN command.