“Additional U.S. strikes will continue to target ISIL in Sirte in order to enable [Libya’s unity government] to make a decisive, strategic advance,” Cook said, using an acronym for the Islamic State. It was the first U.S. strike against the Islamic State in Sirte.

Speaking in a video statement, Fayez Serraj, prime minister of Libya’s Western-backed unity government, said he had requested the assistance to help the advance of local forces in Sirte, but said outside military involvement would remain limited in his country’s fight against the Islamic State.

If the use of American air power is sustained, the Sirte campaign would open a new chapter in the Obama administration’s war against the Islamic State and its campaign to establish a caliphate across a wide swath of Africa, the Middle East and Asia. While U.S. and allied war planes have been conducting strikes for two years in Iraq and Syria, its 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 2015 attack on the group’s leader there.

The United States is also conducting periodic strikes on Islamic State targets in Afghanistan.

The attacks come at a pivotal point in the effort to retake control of Sirte, which militants seized in early 2015 and have sought to transform into an annex of its so-called caliphate. Since May, pro-government forces, led by militias based out of Misrata, have targeted the extremists from three sides of the city, as well as a naval blockade to prevent fighters from fleeing. Within weeks, the militias liberated large sections of the seaside city.

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But the Islamic State fighters have managed to keep a stiff resistance, deploying snipers and roadside bombs and setting up booby traps for the pro-government militias, slowing their advance and preventing them from asserting control of the whole city.

Today, 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 Gaddafi built to host African Union gathering and other events.

The militants have also used Sirte as a base of operations to seize more territory and launch attacks on Libya’s oil infrastructure. Sirte, Gaddafi’s hometown, is strategically nestled on the fractured 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 barrels of oil and regain a measure of economic stability.

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Mohamed Eljarh, a scholar with 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 pro-government fighters had been killed fighting for Sirte, a heavy toll for a relatively small force.

Libyan officials have said that the Sirte operation has been slowed by a lack of equipment and expertise needed for dealing with the improvised explosives that militants have laid across Sirte.

Serraj’s insistence in his remarks on 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 also dealing a blow to the Islamic State.

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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 Gaddafi.

For months, small teams of U.S., 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 of the array of militia forces are potential partners.

Serraj said foreign assistance on the ground would consist of logistical and technical support only. “We repeat from here our rejection of interference by any nation or attempts to violate Libya’s sovereignty,” he said.

The prime minister’s announcement, which was made before the Pentagon acknowledged the strikes, may serve to bolster the perception that his government is acting with strength and international backing. “In another way, it could backfire against him in Libya,” Eljarh said.

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He said that opponents of Serraj’s unity government, including those allied with a parallel legislature based in the eastern city of Tobruk, would likely seize on the fact that Serraj made his request before his government had secured full backing as required under the U.N. political process.

Serraj came to power earlier 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 pro-government and Islamic State forces have been unfolding daily. Sources in Misrata said the airstrikes targeted Islamic State vehicles, possibly carrying weapons. U.S. officials said the strikes were conducted by manned and unmanned aircraft and hit targets including a T-72 tank.

Thomas Gibbons-Neff contributed to this report from Ankara. Raghavan reported from Cairo.