A new intelligence assessment shows the size of the Islamic State force is shrinking in Iraq and Syria — but growing in Libya, a defense official said.

The new estimate for the number of fighters for the Islamic State group, also known as ISIS or ISIL, is between 19,000 and 25,000 in Iraq and Syria, down from prior figures ranging from 20,000 to possibly more than 30,000, according to a defense official who spoke on condition of anonymity.

The exact reasons for the declining force size are unclear but may include battlefield deaths, the impact of U.S. airstrikes, desertions and recruiting problems, the defense official said.

Meanwhile, U.S. intelligence officials believe there are 5,000 to 6,000 ISIS fighters in Libya, up from previous estimates of 2,000 to 3,000.

"It is getting harder to get into Syria for foreign fighters and a lot of them are diverting to Libya," the defense official said.

The revised estimate comes as the White House is reportedly considering an expansion of anti-ISIS operations and airstrikes into Libya.