Correction, 2/8: An earlier version of this story said Libya's unity government was called the Government of National Authority. This was incorrect. It is the Government of National Accord.

US fighter jets have carried out air raids on positions of the Islamic State of Iraq and the Levant (ISIL) in the Libyan city of Sirte for the first time, according to the country's unity government.

"The first American air strikes on precise positions of the Daesh (ISIL) organisation were carried out today, causing heavy losses ... in Sirte," Prime Minister Fayez al-Sarraj said in a speech on television on Monday.

The Pentagon said the raids were launched in response to a request from the unity government, known as the Government of National Accord (GNA).

Al Jazeera's Rosiland Jordan, reporting from Washington DC, said the Libyan army had asked for air support after facing booby traps, mines, and roadside bombs in and around Sirte.

"ISIL made it physically very difficult to follow them as ISIL strengthens their grip on the heart of the city," she said.

"These air strikes are a way of clearing the terrain and making it safer for Libyan troops to advance."

The battle will become more complex as it moves closer to the centre of the city because there are many civilians there who could get caught in the crossfire, Jordan said.

Gaddafi home town

Speaking to reporters, Pentagon press secretary Peter Cook said the US would continue to carry out air raids in coordination with the GNA.

"The specific targets will be precision targets," Cook said. "One of the targets struck today was a tank ... the United States military will be rigorously involved in every step of the process.

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"We don't have an end point [for the bombing campaign] at this particularly moment in time ... we certainly hope that this is something that does not require a lengthy amount of time."

The Tripoli-based GNA in May launched an assault to retake Sirte, the hometown of former Libyan leader Muammar Gaddafi, which ISIL has controlled since June 2015.

The fall of Sirte, 450km east of Tripoli, would be a major blow to the group, which has also suffered a series of setbacks in Syria and Iraq.

The battle for the city has killed around 280 pro-government fighters and wounded more than 1,500, according to medical sources attached to government forces.

The pro-GNA forces are mostly made up of militias from western Libya established during the 2011 war that overthrew Gaddafi.

A militia set up to guard the country's main oil facilities has also been advancing on ISIL.

The GNA was formed as part of a UN-brokered power-sharing agreement reached in December, but it has yet to be endorsed by Libya's elected parliament based outside Tripoli in the country's east.