US ambassador Nikki Haley said the force's authorized strength of 19,815 would be cut by 3,600 -- but that 3,100 of these places are already unfilled.
The central African giant is under pressure to hold key elections this year, despite ongoing fighting between armed factions, political chaos and widespread corruption.
But UN member states, in particular President Donald Trump's new US administration, want to scale back UN spending on its missions around the globe.
And Haley said it was more important to pressure President Joseph Kabila's government to honor political agreements than to bolster the UN force.
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In practice, this will mean 500 fewer troops on the ground, since the mission, known as MONUSCO, was already not at full strength. UN police numbers are unchanged.
"You have to look at the political side of the peacekeeping mission," Haley said.
"We're trying to get aid to the people on the ground, and the government is not letting us. Adding more troops will not change that problem."
"All we're doing is reducing 500 troops," she said, promising the whole strategy of the mission would be reviewed and the Congolese government held accountable.
But France's UN ambassador pronounced himself satisfied with the draft resolution.
"You know it is the outcome of an important negotiation. And so if things go well we will go for a vote tomorrow at noon," Francois Delattre told reporters.
"And I think it is important for DRC, for MONUSCO and for peacekeeping in general. This is a good agreement."
On Wednesday, Haley said she would use her presidency of the Security Council next month to launch a review of peacekeeping.
"What was the original intent of the mission? Is the mission achieving its objective? Do we have an exit plan and is there accountability?"
Meanwhile, the situation on the ground in the DRC is tense.
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