An intense fightback by the Islamic State group in Libya's Sirte slowed an advance by pro-government forces Monday into the jihadists' North African stronghold.

Forces allied with Libya's unity government are closing in on Islamic State group fighters in Sirte in a month-long operation aimed at ousting the jihadists from their North African stronghold.

Forces allied with Libya's unity government battled Sunday to retake the Islamic State group's last redoubts in its stronghold of Sirte, facing fierce resistance including a series of suicide car bombings.

Fighting raged for control of the Islamic State group's Libya stronghold of Sirte after unity government forces on Thursday battled their way into the center of the city.

The prime minister of Libya's U.N.-backed unity government has ruled out an international military intervention to fight the Islamic State group, which has had a growing presence in the country since 2014.

Some 25 nations including the United States and Russia agreed last month to help Libya arm itself against the jihadists, but Prime Minister Fayez al-Sarraj told French newspaper Journal du Dimanche he would not allow foreign troops on the ground.

The foreign minister of Libya's unity government on Monday formally moved into his ministerial offices in the capital Tripoli, as the Government of National Accord struggles to assert its authority.

Mohamad Taher Siala, a foreign ministry employee for three decades, "officially took up his post" at the ministry building, Libyan news agency LANA reported.

At least five migrants have drowned off the Libyan coast after the heavily overcrowded boat they were sailing on overturned, the Italian navy said Wednesday, revising downwards an earlier death toll.

The navy said at least 550 people had been pulled to safety but added that the rescue operation was still underway and that the death toll could rise.

Libyan coastguards said they detained 550 people trying to reach Europe illegally by boat on Tuesday, the second time in three days they have intercepted migrants in the same area.

"Coastguards in the west who were patrolling off the Zawiya refinery on Tuesday intercepted four large inflatables carrying around 550 illegal migrants," navy spokesman Colonel Ayoub Qassem said.

Libya's unity government urged the international community Thursday to accelerate the arming of its forces after 32 of its fighters were killed in clashes with jihadists and a car bombing.

The United States, Italy and Libya's allies and neighbors agreed in Vienna on Monday to exempt the Government of National Accord (GNA) from a U.N. arms embargo to help it confront the threat from the Islamic State (IS) group.

The African Union on Wednesday threw its weight behind Italian and U.S.-led moves to stabilize Libya that include arming a national unity government to fight Islamic State militants.

Western powers are also keen to see Prime Minister Fayez al-Sarraj's fledgling administration establish its authority over the whole country so it can actively contribute to efforts to stem the flow of mostly African migrants using Libya as a launchpad to get to Europe.