Militants with the Islamic State group killed five Islamist-backed militia fighters in an attack on a checkpoint Wednesday in the Libyan coastal city of Sirte, a local official said.

The militia fighters are part of the so-called Brigade 166 tasked by the Islamist-backed Tripoli government with securing Sirte.

They have engaged in sporadic fighting with IS jihadists around the city, the home town of slain dictator Moamer Kadhafi, whose regime was toppled in a 2011 NATO-backed revolt.

"Islamic State jihadists attacked a checkpoint 15 kilometres (nine miles) west of Sirte, killing five members of Brigade 166 belonging to the Libya Dawn group and tasked with protecting the city," brigade spokesman Khaled Abou Jizya said.

"Fierce clashes erupted in the early morning and the Islamic State fighters were repelled," Abou Jizya added.

A local official told AFP that IS fighters used rockets in their attack.

In an audio message posted online IS meanwhile claimed responsibility for a double suicide car bombing that killed at least seven soldiers in Libya's second city of Benghazi on Tuesday.

Last week at least 12 Libya Dawn militiamen were killed in clashes with IS jihadists near Sirte.

Libya has been plagued by chaos since the end of the 2011 revolt that toppled Kadhafi, with heavily armed militias battling for control of its cities and oil wealth and rival governments and parliaments vying for power.

The country has had two governments and parliaments since Tripoli was seized in August by Libya Dawn and the internationally recognised government fled to the country's far east.

Sirte, 450 kilometres (280 miles) east of Tripoli, has been a bastion of Islamic extremism, with rival groups positioned in the Mediterranean city.