The battle for Sirte, the stronghold of ISIL Takfiri group in Libya, raged Thursday after unity government forces pushed into the centre of the city and predicted it could fall within days.

The battle for Sirte, the stronghold of ISIL Takfiri group in Libya, raged Thursday after unity government forces pushed into the centre of the city and predicted it could fall within days.

The United States welcomed the advance on Sirte, the hometown of ousted dictator Moamer Kadhafi, which has also been in the sights of forces of a rival authority in eastern Libya.

The loss of Sirte would amount to a huge setback for ISIL, which is also faced with battlefield losses in Syria and Iraq.

"The armed forces entered Sirte. They are currently in the centre, where clashes continue with Daesh," said Mohamad Ghassri, spokesman for the forces of the UN-backed Government of National Accord (GNA), using another name for ISIL.

"The operation will not last much longer. I think we'll be able to announce the liberation of Sirte in two or three days," he told AFP.

Brett McGurk, US President Barack Obama's special envoy to the international coalition fighting ISIL, confirmed the advance.

"GNA aligned forces now making rapid advances against ISIL in Libya and beginning to enter its stronghold in Sirte," he tweeted.

Tightening the noose on ISIL, air strikes targeted the area around a conference centre where the insurgents had set up a command post, while the GNA's navy said it was in control of the waters off the eastern city.

A US Defense Department spokesman welcomed the advance.

"We certainly are encouraged by the progress we see those government forces making," Pentagon press secretary Peter Cook said.

"The fact that they are now under pressure in Libya... we think is a good thing and suggests that the Government of National Accord and the forces supporting that government are making progress."

Aziz Issa, a hospital spokesman in Misrata, east of Tripoli, said 115 militants had been killed and 300 wounded in the anti-ISIL assault since mid-May.

Stepping up the operation, the GNA's navy has taken control of the coast of Sirte as part of the offensive, said Rida Issa, its commander for central Libya.

"Our forces control the entire coast of Sirte. They (ISIL militants) will not be able to flee by sea," he told AFP.

Naval forces had supported the offensive, he said, including by "carrying out operations to open the way for ground forces to advance along the coast".

On Wednesday, the unity government said its forces had captured two barracks from the Takfiris near Sirte, which IS has held since 2014.