WASHINGTON — The United States has launched airstrikes on the Islamic State stronghold in Sirte, Libya, the Pentagon said Monday, the first direct US involvement in the fierce battle unfolding there and a significant expansion of the American campaign against the group.
In a statement, Pentagon press secretary Peter Cook said Libya’s Western-backed unity government had requested the air support as forces under its command battle to reclaim the coastal city of Sirte, which became an important Islamic State stronghold after militants seized it last year.
‘‘Additional US strikes will continue to target ISIL in Sirte in order to enable [Libya’s government] to make a decisive, strategic advance,’’ Cook said, using an acronym for the Islamic State.
It was the first US strike against the Islamic State in Sirte.
In a video statement, Fayez Serraj, prime minister of the unity government, said he had requested the assistance but said outside military involvement would remain limited.
If the use of US air power is sustained, the Sirte campaign would open a new front in the Obama administration’s war against the Islamic State and its campaign to establish a caliphate across a swath of Africa, the Middle East, and Asia.
While US and allied war planes have been conducting strikes for two years in Iraq and Syria, actions against the group’s Libya affiliate, which officials have described as its most powerful branch, have been limited to a small number of targeted airstrikes since last year, including a November attack on the group’s leader there.
The United States also conducts periodic strikes on Islamic State targets in Afghanistan.
The attacks come at a pivotal point in the effort to retake Sirte. Since May, progovernment forces, led by militias from Misrata, have targeted the extremists from three sides of the city and used a naval blockade to prevent fighters from fleeing. Within weeks, the militias liberated large sections of the city.
But Islamic State fighters have offered stiff resistance, deploying snipers and roadside bombs and setting up booby traps for the militias, slowing their advance and preventing them from asserting control of the whole city.
A few hundred militants are believed to be holed up in a few sections of the city, mainly in a sprawling conference hall complex that former Libyan dictator Moammar Ghadafi built.
The militants have also used Sirte as a base of operations to seize more territory and launch attacks on Libya’s oil infrastructure.
Sirte, Ghadafi’s hometown, is strategically nestled on the country’s oil crescent, where much of its petrochemical operations are located. Liberating Sirte could allow Libya, in the throes of a financial crisis, to pump and export more oil and regain a measure of economic stability.
Mohamed Eljarh, a scholar at the Atlantic Council, a Washington think tank, said the request from Serraj was a response to the intensity of the Islamic State’s defense of Sirte.
According to Serraj’s government, which is backed by Western nations but has been struggling to win wider backing among Libyans, at least 250 progovernment fighters had been killed fighting for Sirte, a heavy toll for a relatively small force.
Libyan officials have said the Sirte operation has been slowed by a lack of equipment and expertise for dealing with the improvised explosives that militants have laid across Sirte.
Serraj’s insistence in his remarks Monday that foreign military involvement would remain limited to a support role is an indication of the delicate course he must maneuver as he seeks to shore up his government’s legitimacy while dealing a blow to ISIS.
The protests that followed the death last month of three French troops in eastern Libya, which forced the French government to acknowledge its military presence, illustrated the sensitivity of foreign military involvement in Libya five years after the NATO-led intervention that toppled Ghadhafi.
For months, small teams of US, French, and British forces have been on the ground in Libya, keeping a low profile as they gather information about the militant threat there and identify which militia forces are potential partners.
Serraj said foreign assistance on the ground would consist of logistical and technical support only. His announcement, made before the Pentagon acknowledged the strikes, may bolster the perception his government is acting with strength and international backing.
‘‘In another way, it could backfire against him in Libya,’’ Eljarh said.
He said opponents of Serraj’s unity government, including those allied with a parallel legislature based in the eastern city of Tobruk, are likely to seize on the fact that Serraj made his request before his government had secured full backing, as required under the UN political process.
Serraj came to power this year following lengthy negotiations brokered by the United Nations that sought to end Libya’s political schism. Since 2014, there have been multiple rival regimes in Libya, each one claiming sole legitimacy.
According to early reports from Libya, the assault unfolded in a front-line section of the city, where skirmishes between the progovernment and Islamic State forces have been unfolding daily.
Sources in Misrata said the airstrikes targeted Islamic State vehicles, possibly carrying weapons.
US officials said the strikes were conducted by manned and unmanned aircraft and hit targets that included a T-72 tank.
