CAIRO - The Pentagon has announced that announced that, at the request of the United Nations-backed Libyan government, the U.S. has begun conducting airstrikes against the Islamic State of Iraq and Syria (ISIS) in Libya.

Defense Department spokesman Mark Wright said Monday that U.S. forces conducted precision air strikes against ISIS targets in Sirte, to support forces affiliated with the Libyan Government of National Accord (GNA).

"These strikes were authorized by the president following a recommendation from Secretary Carter and Chairman Dunford," Wright said. "They are consistent with our approach to combating [ISIS] by working with capable and motivated local forces."

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Wright said that GNA-aligned forces have previously recaptured territory around Sirte from ISIS. He said there will be additional U.S. airstrikes to enable the GNA to make "a decisive, strategic advance," and to deny ISIS a safe haven in the country.

The strikes mark the start of a more intense American role in the fight against ISIS in Libya, as the U.S. steps in to assist the fragile, U.N.-backed government there.

CBS News correspondent Margaret Brennan reports that, until four to five months ago, the U.S. had been unwilling to strike in Libya before a government was in place.

A senior administration official told CBS News that the growth of ISIS was very fast, increasing quickly "like a hockey stick" too close to Europe.

In June the U.N. Security Council unanimously adopted a resolution allowing European Union ships off the Libyan coast to stop vessels suspected of smuggling arms and seize any illegal weapons.

Over the past six weeks, forces fighting for the so-called GNA began moving into the ISIS stronghold at Sirte, on the central coast.

"They've been having real success," Brennan said of the Libyan forces, "and the U.S. had been looking for ways to help them."

Brennan notes that President Obama, and the Libyan Prime Minister, had authorized the strikes to begin earlier, but held off because of a recent fatal incident involving French Special Forces. On Thursday, Mr. Obama's ISIS team met, after which the Libyans gave the green light.

In a televised statement Monday Fayez Serraj, the head of the U.N.-brokered presidency council, said that the presidency council had made a request for direct U.S. airstrikes, but that no U.S. ground forces were involved.

Serraj said today's airstrikes in Sirte caused "major casualties."

Sirte was the hometown of former Libyan strongman Muammar Qadaffi. It was there that he was captured and killed in October 2011, amid the Arab Spring uprisings that swept through parts of the Middle East.