The U.N. envoy to Libya says attacks by the Islamic State group are a serious threat to Libya's oil installations.

Martin Kobler says in a statement Wednesday that he is deeply concerned by recent IS incursions into Libya's oil crescent region and militant attacks on oil fields, calling it a "grave assault" on the economy and "the livelihoods of millions of ordinary Libyans."

Libya slid into chaos following the 2011 uprising that toppled Moammar Gadhafi and has been split between rival governments since 2014.

IS militants gained a foothold in Libya amid the power struggle and security vacuum. It's taken over the central city of Sirte and carried out deadly attacks across the country.