A 12,600-strong force officially replaced the AFISMA military mission, which has been supporting French soldiers who entered Mali in January to halt an Islamist advance and help the government re-establish its authority over the vast country.
Soldiers from nine troop-contributing west African countries and UN officials took part in a "re-hatting" ceremony in Bamako on Monday, replacing their head gear with the distinctive light blue berets of the UN.
France is winding down its deployment from its peak of nearly 4,500 but is to keep up to 1,000 troops in Mali and will maintain responsibility for military strikes against the Islamists.
The force is made up largely of Africans already stationed in Mali but China has offered to supply more than 500 troops in what would be its biggest contribution to UN peacekeeping.
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The mission is due to play a key role in presidential polls announced for July 28 but the election commission has raised doubts over its ability to stage a free and fair vote with such short notice.
The commission's president Mamadou Diamountani said last week it would be "extremely difficult" to get up to eight million voting cards to the electorate in a country where 500,000 people have been displaced by conflict.
But the ethnic-Tuareg National Movement for the Liberation of Azawad (MNLA) in Kidal said Saturday that "nothing justified delaying" the poll.
Malian military officers staged a coup in March last year, but the weak army was overpowered by the MNLA which seized key northern cities before being sidelined by its Al Qaeda-linked allies.
The MNLA sided with a French-led military intervention which reclaimed most of the lost territory from the Islamists.
UN officials have acknowledged that the peacekeepers face the threat of guerrilla attacks and will encounter a number of logistical difficulties in northern Mali's harsh environment, where water is scarce and temperatures sore above 40 degrees Celsius.