The U.N. says an armed group has stormed the main water distribution station in Libya's capital and closed valves supplying Tripoli and cities in the northwest, potentially affecting some 2 million people.

U.N. spokesman Stephane Dujarric said Monday that U.N. humanitarian officials are deeply concerned over the reported water cutoff.

He said some Tripoli districts are already experiencing low water pressure, "and it is expected that the full impact will be felt in the coming two days unless the valves are reopened."

Libya plunged into chaos after dictator Moammar Gadhafi was killed in 2011.

The self-styled Libyan National Army led by Khalifa Hifter launched an offensive from the east on April 4 seeking to take Tripoli. Hifter's forces are battling militias loosely allied with the weak, U.N.-recognized government in Tripoli.