Libyan pro-government forces, backed by US air strikes, fought to advance on Islamic State group jihadists in Sirte on Wednesday despite mines and snipers, a spokesman said.

"Our forces... are trying to strengthen their advance with the support of ongoing American air strikes that have given momentum to the military operation," said Reda Issa, a spokesman for forces loyal to Libya's unity government.

The IS bastion, located just across the Mediterranean from Europe, has been shaken by weeks of fierce clashes between jihadists and fighters allied to Libya's UN-backed Government of National Accord (GNA).

American war planes carried out seven strikes against IS positions in Sirte on Monday and Tuesday at the GNA's request.

Issa did not say whether further strikes took place on Wednesday.

GNA forces have been battling to oust jihadists from the town since May 12. They entered the city on June 9 and have pushed the jihadists out of the city's port, international airport, an air base and a hospital.

But their advance slowed as IS hit back with sniper fire, car bombs and suicide attacks.

"There are targets that are hard to hit because they are among the houses," said Issa.

"American air strikes, which are very accurate, will help to destroy those targets."

"Every day the battle's outcome is not settled, the city becomes more full of booby traps," said Issa.

"But there is no doubt that the presence of effective and accurate weapons will accelerate the end of the battle."

US President Barack Obama defended the air campaign on Tuesday, saying defeating the jihadists there was in America's national interest.

The loss of Sirte, the birthplace of Libya's former dictator Moamer Kadhafi, would be a major blow to the jihadists, who have faced a series of setbacks in Syria and Iraq.