The Islamic State (IS) militant group is growing weaker in Iraq and Syria but has gained strength in Libya, according to a new U.S. intelligence assessment.
The new estimates, revealed by the White House on February 4, found that the number of IS fighters has fallen to about 25,000 in Syria and Iraq, the group's so-called "caliphate," down from a previous estimate of about 31,000.
The roughly 20 percent decline was due to battlefield casualties and desertions, as well as progress in the U.S.-led campaign to crush IS through a combination of air strikes and supporting Iraqi and Kurdish forces on the ground.
"ISIS has sustained significant casualties," said White House spokesman Josh Earnest, using another widely known acronym for the extremist group.
He gave credit to ground-fighting efforts by Syrian opposition groups as well as coalition partners, including Iraq's military and tribal forces and Kurdish Peshmerga troops. No mention was made of Russian air strikes directed in part at IS since September.
The U.S.-led air campaign, which has hit IS with more than 10,000 strikes since 2014, has had an impact, as have international efforts to stem the flow of foreigners seeking to join the movement, Earnest said.
"[IS] is having more difficulty than they've had before in replenishing their ranks," he said.
The U.S. intelligence community now estimates that IS troops number between 19,000 and 25,000 in Iraq and Syria, down from 20,000 to 31,000 in 2014.
"The decrease reflects the combined effects of battlefield deaths, desertions, internal disciplinary actions, recruiting shortfalls, and difficulties that foreign fighters face traveling to Syria," said Emily Horne, a spokeswoman for the National Security Council.
But IS's dwindling numbers in the Middle East may also reflect in part some North African militants heading to Libya rather than traveling to Syria to join the group as they did in previous years, officials found.
IS has taken advantage of political chaos in Libya to expand its grip on territory around Sirte and the Mediterranean coast. U.S. estimates of IS forces in Libya now range between 3,000 and 6,000, about double what they were in previous years.
The Pentagon has been monitoring IS growth in Libya and is developing a range of possible U.S. responses at the White House's request, including the option of taking military action.
U.S. Defense Secretary Ash Carter has said he doesn't want to see Libya become like Iraq or Syria, where IS took advantage of a vacuum created by political chaos and internal conflict to quickly gain control over wide swaths of territory in 2014.
The United States has already taken some limited steps in Libya. In November, a U.S. air strike killed top IS leader Abu Nabil, an Iraqi.
And in December, the Pentagon acknowledged that a group of U.S. special operations troops who traveled to Libya to "foster relationships" was kicked out of the country soon after arriving.
"We're going to continue to watch how the threat in Libya evolves and we're going to continue to be prepared to take action," Earnest said.