“All the city’s residential areas were planted with mines and explosives,” said General Mohamed Geneidi, a commander of the forces allied with Libya’s UN-backed unity government.
“Removing (them) will take some time,” he told AFP. “We hope this will be completed… within a month and a half.”
Forces backing the Government of National Accord [GNA] announced they had taken full control of Sirte on Monday, in a major blow to IS extremists after almost seven months of fighting.
But parts of the city remain no-go areas.
Geneidi said IS left bombs “on the sides of houses, in gardens, on doors, in fridges, even the furniture. We found explosives in kitchens and bedrooms.”
GNA forces have only basic mine-clearing equipment, he said.
IS seized Sirte in June 2015, taking advantage of the chaos following the 2011 fall of former dictator Muammar Gaddafi.
GNA forces launched an assault in May to wrest the coastal town, 450 kilometres (280 miles) east of Tripoli, from extremist control.
IS put up a fierce defence, using snipers, suicide car bombings and improvised explosive devices.
That slowed the operation to a grim battle of attrition that left nearly 700 GNA fighters dead and 3,000 wounded.
It is not known how many IS fighters died, but pro-GNA forces said on Thursday that more than 260 bodies of fallen extremists had been found in 48 hours on roads and in buildings of Sirte.