Wednesday evening's attack on a highway checkpoint in the desert interior comes after the jihadists thrust west along the Mediterranean coast from their stronghold of Sirte last week, overrunning a major crossroads.
The checkpoint at Saddada lies 50 kilometres west of the Abu Grein crossroads and marks a new advance into territory held by forces loyal to the unity government in Tripoli.
"Two suicide bombers, one in a vehicle and one on a motorbike, blew themselves up at the checkpoint where troops had gathered and clashes then broke out between our forces and IS fighters," a spokesman for the anti-IS operations command told AFP.
Libya's LANA news agency said the ensuing fighting lasted six hours.
Abu Grein, where the highway along the Mediterranean meets the main road south into the desert interior, lies 120 kilometres south of Misrata and its capture by IS prompted militia in Libya's third city to mobilise.
Saddada is just 100 kilometres from Misrata.
It is 190 kilometres from Sirte, the hometown of slain dictator Muammar Gaddafi which IS overran in June last year and has since transformed into a training camp for Libyan and foreign militants.
With its port and airport, there are fears the jihadists could use the city as a staging post for attacks on European soil.
The IS capture of the Abu Grein crossroads on June 5 was its first expansion to the west of Sirte and has led to hundreds of families taking flight from the neighbouring town of the same name.
The jihadist group controls zones to the east of its Mediterranean bastion.
The group is estimated to have about 5,000 fighters in Libya, and it is trying to attract hundreds more.
Western powers including the US, Britain and France have openly considered international military intervention in Libya against IS.
They have expressed strong support for the new unity government which has slowly asserted its authority in Tripoli since the end of March.
