The White House is extending its authorization for military strikes against Islamic State targets inside Libya, where militants are fiercely clinging to a small patch of territory along the Mediterranean Sea, a U.S. defense official said Monday. 

The mission there, known as Operation Odyssey Dawn, was expected to last only weeks when it began in early August. But after more than three months and at least 367 airstrikes, about 200 ISIS fighters continue to control a neighborhood in the city of Sirte.

"They are fighting to the death it appears," Pentagon spokesman Peter Cook said during a press briefing. "There is still a fight going on in this one particular neighborhood. There's a small group of [Islamic State] fighters who remain," Cook said.

U.S. Africa Command had not reported any airstrikes in Libya since Oct. 31, prompting speculation that the American military was quietly ending its air campaign there. But the Pentagon says it still has authority to launch airstrikes in support of allied militias loyal to Libya’s fledgling Government of National Accord, or GNA.

"If additional airstrikes are needed," Cook said, "we will be prepared to deliver those strikes."

This Islamic State faction's ability to hold ground highlights the challenge faced by the U.S. and its allies in trying to prevent the terror group from exploiting lawless regions like those inside Libya, which collapsed into civil war in 2011 after the fall of its leader Muammar Gaddafi.

The fight there raises concerns about the far larger operation against ISIS in Iraq, where the U.S. military provides airstrikes, special operations troops on the ground and other military support for Iraqi forces.

Cook sought to highlight gains in Libya since the start of the U.S. air campaign. Earlier this year, U.S. intelligence officials estimated Libya had as many as 6,000 fighters loyal to the Islamic State, also known as ISIS or ISIL.

"It was not too long ago that ISIL controlled Sirte. And that has changed. That has changed because of the work of the GNA-aligned forces and the support that we've been able to provide from the air," Cook said.

A defense official familiar with the operation said the White House only recently extended the authorization for Libya, which is tied to a 2001 vote in Congress that approved military action against al Qaida. Officials declined to say precisely when the authority was extended or for how long.