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DAKAR (Reuters) - Militants on Thursday killed three United Nations peacekeepers in a attack outside their base in Kidal in northern Mali, the U.N. said in a statement on Friday.

U.N. soldiers face frequent attacks from desert fighters who have regrouped since a French-led military operation in 2013 to drive them out of Mali’s northern towns.

More than 100 peacekeepers have died in recent months, making it the most deadly U.N. mission to date.

The soldiers were from Guinea, the Guinean government said in a statement. The identity of the attackers was not yet known.

The U.N. camp came under heavy mortar fire on Thursday, injuring five. Shortly afterwards, three U.N. soldiers were killed in an attack just outside the camp, the U.N. said, giving no further details.

Despite ongoing French strikes on their hideouts, fighters, including local al Qaeda affiliates, have staged a series of attacks this year, including a suicide bombing at an army base in January which killed at least 77 people.

Several French soldiers were wounded last week in an al Qaeda mortar attack in Timbuktu.