US Special Operations forces are providing direct, on-the-ground support for the first time to fighters battling the Islamic State in Libya, US and Libyan officials said, coordinating American airstrikes and providing intelligence information in an effort to oust the group from a militant stronghold.

The positioning of a small number of elite US personnel in the coastal city of Sirte deepens the involvement of Western nations against the Islamic State's most powerful affiliate.

US officials, who spoke on the condition of anonymity to discuss a mission that has not been announced publicly, said that US troops are working out of a joint operations centre on the city's outskirts and that their role is limited to supporting forces loyal to the country's fragile unity government.

Robyn Mack, a spokeswoman for US Africa Command (AFRICOM), said small numbers of US military personnel will continue to go in and out of Libya to exchange information with local forces but declined to provide details.

An expanded on-the-ground role for Western nations follows President Barack Obama's administration's decision earlier this month to begin regular airstrikes on Islamic State positions in Sirte, the group's de facto capital in North Africa. Since the strikes began about a week ago, US planes have struck almost 30 militant targets.

The increased US air campaign against the Islamic State in Libya underscores the stakes in a battle against a group that has vowed to strike the West and has attracted recruits from across Africa and the Middle East. Since they appeared in Libya in 2014, fighters allied with the Islamic State have displayed tactics similar to their parent group in Syria and Iraq: beheading non-Muslims, attacking local security forces and facilities associated with Westerners, and forcing locals to abide by their harsh interpretation of Islam.

The new American operation in Sirte is the culmination of an extended, low-visibility mission in Libya by US special operators, who established small outposts in recent months as part of an effort to build ties with friendly forces and increase American understanding of the complexities of political and militia factions. Previously, US troops were focused on holding talks with an array of militia factions to identify potential partners and gathering information about the situation on the ground, including the threat from the Islamic State.

The limited nature and size of US operations around Sirte reflect the delicate balancing act the Obama administration must achieve as it seeks to help allied local forces succeed while not undermining the country's fragile unity government. Last month, Libyans protested France's military footprint in eastern Libya after the death of French troops revealed their presence there.

Even in recent days, Libyan militia commanders have declared that there were no Western boots on the ground and that this was their fight alone. The pro-government forces in Sirte are mostly militia fighters from the city of Misurata, about 240 km by road to the northwest.

Mattia Toaldo, a Libya expert at the European Council on Foreign Relations, said the US mission in Sirte differed from the French presence in the eastern city of Benghazi, mainly because none of Libya's feuding political factions would object to attempts to defeat the Islamic State.

"As long as they keep this low profile. . . the risks both for the US and for the Libyan government are quite low," he said.

But even with the aid of US airstrikes, pro-government forces have found it challenging to push into militant territory as they face an array of obstacles, including land mines, snipers and booby-trapped buildings. As US munitions hit extremists' military vehicles and mobile ammunition depots, the militants adapted by reducing their visibility and hiding in tanks, armored personnel carriers and rocket launchers.

On Monday, US fighter jets could be heard zipping over Sirte, and there were loud explosions inside militant areas. According to AFRICOM, those strikes hit multiple fighting positions and a truck.

At least five pro-government fighters were killed and dozens more were injured in heavy fighting in the al-Dollar neighbourhood this week. The wounded included several top front-line commanders, Libyan militia sources said. US officials said that American forces are not taking part in combat or even directly acting as spotters for airstrikes, and that no Americans have been wounded.

Also this week, US and British personnel, carrying radios and wearing black body armor and tan fatigues, were seen within Sirte, according to officers allied with the Libyan government and Western security personnel in the area.

Washington Post