Tripoli (AFP) - 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.
They entered Sirte on Wednesday and have been advancing more quickly than expected against IS, which seized control of the coastal city last year and turned it into its main base of operations in North Africa.
The loss of Sirte would be a major setback for IS, which has also been losing territory in Syria and Iraq where the jihadist group established its self-declared "caliphate" in 2014.
The jihadists are surrounded in a densely populated area of around five square kilometres (two square miles) in the city centre and putting up a stiff fight.
"Three explosions from cars driven by Islamic State suicide bombers targeted our forces in Sirte," Reda Issa, a spokesman for the unity government's forces, told AFP.
Two of the bombers hit gatherings of pro-government forces and another struck at a field hospital, he said.
At least one person was killed and four wounded in the blasts, Issa said.
Pictures published on the Facebook page of the loyalist forces showed the mangled remains of a vehicle and a crater probably caused by one of the blasts.
Several damaged military-type vehicles, some mounted with heavy guns, could also be seen.
Sunday's attacks came a day after pro-government forces said they had recaptured the port in Sirte, the home town of Libya's ousted dictator Moamer Kadhafi, and residential areas in the city's east.
- Heavy street fighting -
The forces are allied with Libya's Government of National Accord, which is backed by the international community as the country's legitimate authority.
The GNA, led by prime minister-designate Fayez al-Sarraj, has been struggling for months to assert its authority in the face of rival administrations vying for power in the chaos of post-Kadhafi Libya.
The pro-GNA forces are mostly made up of militias from western cities, notably Misrata, and the guards of oil installations that IS has repeatedly tried to seize.
They have engaged in heavy street fighting with the jihadists, deploying tanks, rocket launchers and artillery in the fight for the city.
The Misrata militia forces -- who have an arsenal that includes MiG fighter jets and attack helicopters -- have also carried out dozens of air raids against IS.
The operation announced on Sunday on Facebook that it had launched fresh air strikes against IS positions and vehicles in central Sirte.
Much of the fighting has been around a sprawling Kadhafi-era conference centre which once hosted international summits but now houses an IS command centre.
IS has responded to the offensive with machineguns, mortar rounds and sniper fire, as well as car bombings.
A medical official in Misrata said on Saturday that at least 137 pro-GNA fighters had been killed and 500 wounded since the operation began with a sweep towards Sirte on May 12.
- US hails 'rapid progress' -
Foreign intelligence services estimate IS has 5,000 fighters in Libya, but its strength inside Sirte is unclear.
The establishment of an IS bastion in northern Libya, just across the Mediterranean from Europe, raised widespread fears.
Sirte has an international airport and a port and lies just 350 kilometres (220 miles) from the Italian coast.
Foreign powers backing the GNA have welcomed the offensive on Sirte, where IS has carried out atrocities against residents and set up a base to train foreign jihadists.
The UN envoy to Libya, Martin Kobler, said Saturday on Twitter that he was "impressed" by the "rapid progress" of pro-GNA forces.
He also urged "all fighters to respect international humanitarian law," saying "civilians must not be targeted".
Most Sirte residents have fled but officials have said some 30,000 civilians remain in the city.
Washington's anti-IS envoy Brett McGurk also said Friday he was "encouraged by the progress" being made in Sirte, adding there was a good chance IS forces there "could crack pretty quickly".
The United States and Britain are reported to be providing intelligence support for the operation, with the Washington Post reporting earlier this month that a small group of US special forces had been sent to Libya to work with the militias.
Analysts have warned that retaking Sirte would not spell the end of IS in Libya, where the jihadists have fed on political and military divisions since the 2011 uprising that ousted and killed Kadhafi.