SIRTE: Libyan forces said they pushed forward on Tuesday into the last sliver of territory held by Islamic State in its former North African stronghold of Sirte, seizing a number of barricaded houses.

Hours before the push, the forces published video online showing one of their fighters shouting out messages through a loud hailer urging women and children to leave the area and promising them safe passage.

Backed by US air strikes, brigades dominated by fighters from the city of Misrata have been edging forward against militants trapped in an area a few hundred meters wide next to Sirte's Mediterranean coast.

"The Islamic State gangs are trying desperately to resist up until their last breath," said Rida Issa, a spokesman for the forces.

Their six-month campaign has slowed as they have got closer to fully recapturing Sirte, with militants using sniper positions, sand-filled barricades and tunnels to defend their positions against shelling and air strikes.

The Misrata-led forces said there were more than 30 militants among the dead since fighting resumed on Monday, and that they had lost at least three of their own men.

Overnight, they published video footage of one of their fighters on a rooftop overlooking the area, shouting out: "All women and children leave now if you want to live, don't miss this chance."

"Let them go now, there is safe passage, the battle is over," he adds, addressing the militants.

Several groups of civilians, including captured sub-Saharan migrants, have either escaped or been released from the area held by Islamic State over recent weeks. It is not clear how many militants or civilians remain.

As of Saturday, the US had carried out 398 strikes against Islamic State in Sirte, according to a statement from US Africa Command.

Islamic State took full control of Sirte early last year. It expanded its control along about 250 km (155 miles) of Libya's sparsely populated central coastline before local forces began their campaign in May.

Losing Sirte will leave Islamic State with no territory in Libya, though some militants are believed to have escaped in the early stages of the battle.