Nikki Haley, in her first week as US ambassador, has made reform of the United Nations' far-flung peacekeeping operations a top priority, diplomats said.
The missions cost nearly $8 billion a year and Haley said in her Senate confirmation hearing last month that she wants to look at all 16 to see which are succeeding in maintaining peace and which aren't.
"Do we need to shift and do things differently or do we need to pull out?" she asked.
Haley singled out the mission in war-ravaged South Sudan, the world's newest nation, calling it "terrible." She said the government isn't cooperating with the UN force, which has nearly 13,000 troops and police and a current budget of more than $1 billion.
Two diplomats, speaking on condition of anonymity because the conversations were private, said that in discussions this week Haley put a mission-by-mission review of peacekeeping operations as a top priority.
One diplomat said Haiti, where nearly 5,000 UN troops and peace are deployed at an annual cost of about $346 million, is a mission Haley talked about winding up.
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UN spokesman Stephane Dujarric announced on Friday that UN peacekeeping chief Herve Ladsous is leading "a strategic assessment mission" to Haiti next week at the Security Council's request, which will identify "critical needs" in the country and make recommendations on a future UN presence.
A UN peacekeeping official said late Friday that the department is implementing reforms recommended by a high-level panel on peace operations, and is constantly reviewing and adjusting its operations "to stay relevant and cost efficient."
The official spoke on condition of anonymity because he was not authorised to speak publicly.
According to the UN Peacekeeping Department, over 100,000 troops and police are currently deployed in the 16 missions, including nine in Africa.
While some missions have been very successful Haley singled out Sierra Leone others including in the Central African Republic and Congo have been criticised for sexual abuse violations and corruption, and the joint UN-African Union mission in Sudan's Darfur region has been criticised for inefficiency.
Haley stressed that countries contributing troops must hold them accountable for corruption and sexual exploitation, which she said isn't happening.