Loyalists freed a Turkish and an Egyptian detainee as well as 11 Eritrean women "thought to have been held hostage by Daesh", they said in a statement, using an Arabic acronym for IS.
Pro-government forces are fighting the last jihadists holed up in Sirte after launching an offensive to retake the city in May.
Libya has been in chaos with rival administrations and militia vying for power of the oil-rich country since the 2011 uprising that toppled and killed longtime dictator Moamer Kadhafi.
Forces allied with Libya's UN-backed Government of National Unity (GNA) began an offensive on May 12 aimed at ousting IS from the city and surrounding areas.
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Backed by US air strikes since August 1, they have gradually tightened the noose on the few remaining IS fighters inside the city some 450 kilometres (280 miles) east of the capital.
Pro-government forces today said they had seized a group of buildings in a northeastern district of Sirte after three days of fighting against IS diehards.
The GNA -- intended to replace two rival administrations -- is the centrepiece of Western hopes to fight jihadism in Libya and halt people trafficking across the Mediterranean that has led to thousands of drownings.
But the parliament in the far east of the country has failed to recognise it, while its rival administration last week dealt the unity government a new blow when it seized some of its key offices in Tripoli.
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