Libyan pro-government forces say they have captured the headquarters of Islamic State of Iraq and the Levant (ISIL) in Sirte, scoring a major victory in the group's last stronghold in the country.

The fighters, mainly from the nearby city of Misrata, who were supported by US air strikes, said in a Facebook statement on Wednesday: "Sirte is returning to Libya" after taking over a convention centre that served as ISIL's base.

"Our forces have complete control of the whole of the Ouagadougou complex - they even advanced some distance beyond the complex," said Rida Issa, a government spokesman.

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"I am standing inside the walls of the Presidential Palaces, or the so-called Protocol Palaces in Sirte," Al Jazeera's Ahmed Khalifa, reporting from Sirte, said.

"This entire area once under the total control of ISIL fighters is now in the firm grip of Al Bonyan Brigade fighters. The aftermath of fierce fighting seems clearly visible on these buildings."

The militia, which started the offensive in June, said they had also advanced to a cluster of unfinished blocks just west of the centre of Sirte, known as the "bone buildings", which had been used by ISIL (also known as ISIS) snipers.

At least 16 fighters from government-backed forces were killed and 11 wounded, Issa said.

ISIL seized control of Sirte in 2015 [HO/Media centre of the GNA forces military operation against ISIL in Sirte/AFP]

US air strikes

Washington launched its air strikes on August 1, with President Barack Obama saying it was "in America's national security interest" to help the pro-government forces "finish the job" of ousting ISIL from Sirte.

In a statement on Wednesday, the US Africa Command said 29 strikes had been carried out against ISIL positions in the town as part of "Operation Odyssey Lightning" as of Tuesday.

Libya's Government of National Accord operations centre said further US raids were carried out on Wednesday, but did not say how many. The raids targeted ISIL positions, destroyed two armoured vehicles and stopped an explosives-laden car before it could reach loyalist forces, it said.

"During fighting, the US air strikes backing the brigade’s fighters shelled the area destroying a vehicle," said Al Jazeera's Khalifa.

"The vehicle was packed with explosives as prepared for a suicide attack mission.The air strike targeting the area destroyed the vehicle which was surrounded by a number of ISIL fighters.

We can also see the undetonated howitzer-gun projectiles which were inside the vehicle."

READ MORE: Can US air strikes push ISIL out of Libya?

ISIL still controls several residential areas, though, and the Misrata-led brigades have previously found it difficult to advance through neighbourhoods in house-to-house fighting.

Since August 1, US drones and fighter jets have carried out 29 strikes, targeting several ISIL emplacements on Monday and a gun-mounted pick-up truck on Tuesday, according to statements by US Africa Command.

Earlier on Wednesday, Libyan forces said that they had lost a fighter jet over Sirte. Issa said the cause of the crash and the fate of the crew could not be confirmed.

ISIL said it shot down the jet, killing a pilot, according to a statement on a website close to the group.

Post-Gaddafi chaos

Losing Sirte city would be a major setback for ISIL, already under pressure in Syria and Iraq.

It would also be a boost for Libya's UN-backed government, which has struggled to impose its authority and faces continuing resistance from armed group.

ISIL seized control of Sirte, the hometown of Libya's former leader Muammar Gaddafi, in 2015.

The group took advantage of conflict between various factions of former rebels who emerged as powerbrokers after Gaddafi was killed in 2011. 

Source: Al Jazeera And Agencies