A UN strategic review has recommended that its 16,000-strong mission in the Democratic Republic Congo Monusco starts phased exit from the country in "an absolute minimum of three years."

The mission began two decades ago and has an annual budget of over $1 billion but its current mandate comes to an end on December 31.

The UN said an exit at the end of 22 would ensure "a responsible transition" subject to reforms announced by President Felix Tshisekedi on professionalising the army and neutralising rebels remaining on track.

Even if the best-case scenario materialises, the review said a political mission could still be necessary to "prevent a resurgence of conflict."