France has championed the G5 force, which will be made up of troops from Mali, Burkina Faso, Niger, Chad and Mauritania.
It wants donors to step up but it also wants backing from the UN.
The US, which negotiated a 600-million-dollar (500-million-euro) cut in the UN peacekeeping budget this year, says it is ready to give bilateral funding but does not want the UN to finance it.
US criticises lack of clarity
Its UN ambassador Nikki Haley has also criticised the project for being too vague.
Washington wants to know “what the strategy would be, how they see this playing out, what’s involved in it before we ever commit to UN-assessed funding,” she said on Friday.
“Show us something, we’re open to it, we’re not saying no, but what we’re saying right now [is] there literally has been no information that has been given that gives us comfort that they know exactly how this is going to play out.”
The US is the UN's biggest funder but President Donald Trump and the Republican right are hostile to the world body.
The question has become more sensitive since the deaths of four American soldiers in an ambush on a joint patrol with troops from Niger near the Niger-Mali border this month.
“United Nations forces don’t do counterterrorism, they do peacekeeping operations,” General Thomas Waldhauser, who oversees US troops in Africa, said, also on Friday. “One of the hardest things to do in an organisation like that is to try to synchronise the efforts of those five countries and have a coherent strategy as opposed to just a series of engagements in different locations.”
French role in region
UN Secretary-General Antonio Guterres came out in favour of UN support in a recent report that branded the force "an opportunity that cannot be missed".
With chaos in Libya after the fall of Moamer Kadhafi, Islamist insurgency in Mali and the rise of Boko Haram in Nigeria, the Sahel has become a centre for terror groups, smuggling and other lawlessness.
France has had troops in Mali since armed Islamists and Tuareg separatists took control of the north in 2012.
On a visit to Washington last week, French Defence Minister Florence Parly called for American support, saying that France does not intend to become the “Praetorian Guard of sovereign African countries”.
After leading a Security Council visit to the Sahel last week, French Ambassador Francois Delattre said most council members want the UN to help.
"The key question now is not about the relevance of the G5 Sahel force, nor the need to support it, but it is about the best way to convey this support," he said.
With an estimate cost of 423 million euros, only 108 million euros has been raised so far.
French officials say the budget could be reduced to about 250 million euros.
A donor conference is to be held in Brussels on 16 December.