The U.N. special envoy for Libya is urging the country's factions to unite to fight the Islamic State group and stop fighting each other.

Martin Kobler held diplomatic meetings Tuesday in Paris about reinforcing the fledgling U.N.-backed government, as various Libyan forces are advancing on Islamic State strongholds.

He called for "a sound effective structure for the fight ... Libyans should not fight" each other.

Kobler told reporters that defeating IS "requires a military solution." However it "must be a united fight."

Different forces are making unilateral moves against IS in Sirte in central Libya: Military forces from the southeast, pro-UN government militias from Misrata to the west, and other pro-UN government militias from northeastern towns.

IS extremists have taken over territory as rival militias battled for power in Libya.