President Barack Obama defended Tuesday a two-day-old US air campaign against the Islamic State group in its Libyan stronghold of Sirte, saying defeating the jihadists there is in the US national interest.

His comments came as the US military launched a new round of air strikes on the city in the turbulent North African country, following similar strikes Monday at the request of the Libyan Government of National Accord.

"It is in America's national security interest in our fight against ISIL to make sure (the GNA are) able to finish the job," Obama told a White House news conference, using an IS acronym.

"We're working in partnership with them to assure that IS does not get a stronghold in Libya, even as Libya begins what is going to be a long process to establish a functioning government and security system."

Pentagon spokesman Navy Captain Jeff Davis said the United States had conducted seven strikes so far; five on Monday and two on Tuesday.

Two T-72 tanks were among the targets and Davis said some IS fighters had been killed, but he did not have an estimate of how many.

The Tripoli-based GNA launched an operation in May to retake Sirte, the hometown of slain dictator Moamer Kadhafi which has been under jihadist control since June 2015.

While the Pentagon had carried out two previous air attacks on high-value IS targets in Libya, Monday's action marked the first US strikes in Sirte itself, and the first salvos in what the Pentagon hopes will be a quick campaign.

"We don't envision this as something that's going to be too long," Davis said, noting that GNA forces had already done much to dislodge IS fighters from Sirte.

"It's probably going to be weeks, not months."

The GNA's press office said Tuesday that new strikes had occurred against IS positions in Sirte, destroying a rocket launcher and a vehicle.

The IS group had wanted to turn the coastal city of Sirte into its third de facto capital after Raqa in Syria and Mosul in Iraq. The Pentagon estimates fewer than 1,000 fighters remain in the city.

US military officials have insisted operations to support the GNA will be conducted from the air only, with no US boots on the ground.

Still, the Pentagon has previously acknowledged that small teams of special operations forces have been in the country to gain intelligence and build relationships with local forces.

"The August 1 strikes in Sirte indicate that cooperation and coordination has progressed to a level in which all parties are comfortable moving ahead," the US-based Soufan Group security consultancy said.

Authorization to strike Sirte comes from a 2001 law originally aimed at targeting terror groups that supported the September 11 attacks.