They are also buying child soldiers, paying families $600 (£375) per child, Ivan Simonovic said after a fact-finding visit to the country.
‘Extremists’ seized two-thirds of Mali in March when a military coup plunged the country into chaos.
Mr Simonovic painted a grim picture of human rights abuses there.
Women’s rights were being particularly restricted, said Mr Simonovic, the UN Assistant Secretary-General for Human Rights, citing the compilation of a “frightening” list of unmarried women who were pregnant or had borne children.
More women were being forced into marriage – with a wife costing less than $1,000 – and some were then being resold in “a smokescreen for enforced prostitution”, added Mr Simonovic.
“Human rights violations are becoming more systematic,” he told reporters at UN headquarters in New York, adding that Islamists had “imposed an extremist version of Sharia”.
The fact-finding mission gleaned information from people travelling to and from northern Mali, where he said Islamists were imposing harsher punishments for crimes.
So far, he said, there had been three public executions, eight amputations and two floggings.
There were allegations of torture and inhuman prison conditions in southern Mali, where the government retains control, Mr Simonovic added.
He urged authorities to investigate these cases if they expected UN help as Mali’s army tries to reclaim the north.
The 15-member Security Council seems prepared to back an international intervention force in the country, under the right conditions, says the BBC’s Barbara Plett at the UN.
She says council members are deeply concerned about al-Qaeda linked extremists taking advantage of the anarchy in northern Mali.