An official in an operations room set up by the new Government of National Unity (GNA), asking not to be named, told AFP that "the toll is 18 martyrs and more than 20 wounded in the battles with IS militants and a car bombing".
Seven died in a car bomb attack in Buairat el-Hassun, 60 kilometres (35 miles) west of Sirte, the operations room said on its Facebook page. The others were killed in an IS foray in Abu Grein, further west, that the GNA recaptured on Tuesday.
On Monday, the United States, Italy and Libya's allies and neighbours agreed in Vienna to arm the GNA to confront the threat from IS.
Clashes on Tuesday claimed the lives of seven members of the GNA forces and wounded 15.
IS overran Abu Grein on May 5 as part of a series of forays into territory controlled by the UN-backed government, including a deadly attack a week later on a checkpoint at Saddada, 50 kilometres to the west.
The latest fighting comes as IS seeks to expand westwards out of Sirte, slain dictator Moamer Kadhafi's hometown on the Mediterranean coast, which it has controlled since last June.
The international community, particularly European powers, are also concerned about a stream of refugees and migrants crossing the Mediterranean from Libya's unsecured coast.
