By Michelle Nichols

UNITED NATIONS (Reuters) - The United Nations Security Council on Tuesday unanimously authorized a crackdown on arms smuggling in the high seas off Libya by allowing the inspection of vessels to seize and dispose of illicit weapons.

The European Union proposed the council resolution to expand its naval operation in the Mediterranean, which the 15-member Security Council authorized in October to seize and dispose of boats operated by human traffickers.

The council resolution expressed concern that "arms and related materiel are being used by terrorist groups operating in Libya, including ISIL (Islamic State)."

French Ambassador Francois Delattre told reporters ahead of the vote that the resolution has "the potential to be a game-changer," since a large quantity of arms is smuggled via ship off the coast of Libya.

Libya U.N. envoy Martin Kobler told the Security Council last week that Libya was awash with arms - 20 million pieces of weaponry in the North African state of six million people.

"These weapons do not fall from the sky, but come increasingly through illegal shipments by sea and by land. The arms fuel the conflict. These shipments must end if there is any serious hope of bringing peace to Libya," he said.

The U.N. Security Council imposed an arms embargo on Libya in 2011 when former leader Muammar Gaddafi's forces cracked down on pro-democracy protesters.

U.N. sanctions monitors last year told the Security Council that Libya needed help from an international maritime force to halt the flow of weapons in and out of the country.

The fall of Gaddafi in 2011 sparked chaos with two competing governments backed by militias scrambling for control of the oil producing country. A power vacuum has allowed Islamic State militants to gain a foothold.

British U.N. Ambassador Matthew Rycroft said the existing arms embargo had not stopped the flow of arms and that illicit weapons in Libya were "enabling terrorists to murder, to maim, to bring yet more chaos to the people of Libya and the region."

A U.N.-backed unity government formed earlier this year is seen by western states as the best hope for uniting Libya's many political factions. Libya's government is allowed to import arms with approval of the Security Council's sanctions committee.

The resolution asks states to "make good-faith efforts to first seek the consent of the vessel's flag state prior to any inspections" and calls upon those flag states to cooperate.

(Additional reporting by Louis Charbonneau; Editing by Alan Crosby and Andrew Hay)