Its presidential council called on "all military forces" in Libya to await instructions after "a joint command" has been formed and not to launch any unilateral offensive on the IS stronghold of Sirte, 450 kilometres (280 miles) east of Tripoli.
Martin Kobler, the UN special envoy for Libya, said in a tweet: "I welcome the initiative of #Libya Presidency Council to appoint a military joint leadership for operations" against IS.
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The council fears that separate operations in Sirte could spark clashes between the multitude of different fighting forces in Libya and play into the jihadists' hands.
IS has transformed Sirte into a training camp for Libyan and foreign militants since overrunning slain dictator Moamer Kadhafi's hometown on the Mediterranean last June.
With its port and airport, there are growing fears that IS may use Sirte as a staging post for attacks on European soil.