The U.N. envoy for Libya is warning that the oil-rich nation "is on the verge of descending into a civil war" that could divide the country and imperil the security of its neighbors and the wider Mediterranean region.

Ghassan Salame told the Security Council on Tuesday that Islamic State and al-Qaida extremists are already exploiting the security vacuum sparked by the offensive to take the capital Tripoli launched April 4 by the self-styled Libyan National Army.

Salame said "full civil war is not inevitable" — but "it may occur by the will of some parties, and by the inaction of others."

He said "many countries" are providing arms to the warring parties and called on the Security Council to urge an immediate cease-fire and return to a U.N.-led political process.