SYRIA -- A defense official says a new intelligence assessment of ISIS in Iraq and Syria has lowered the estimated number of fighters, reports CBS News' national security correspondent David Martin.
The amount of fighters is now estimated to be around 19,000 to 25,000, down from previous estimates which usually ranged from 20,000 to 30,000 fighters and as high as 33,000.
The drop is attributed to battlefield deaths, trouble recruiting, desertions and increased difficulty getting into Syria, Martin reports.
However, at the same time the number of ISIS fighters in Iraq and Syria is going down, the estimate is going up in Libya -- from 2,000 to 3,000 to 5,000 fighters.
Also on Thursday, Saudi military spokesman told the Associated Press that the kingdom is ready to send ground troops to Syria to fight ISIS if coalition leaders agree during an upcoming meeting in Brussels.
Brig. Gen. Ahmed Asiri told The Associated Press that Saudi Arabia has taken part in coalition airstrikes against ISIS since the U.S.-led campaign began in September 2014, but could now provide ground troops.
The United States is scheduled to convene a meeting of defense ministers from countries fighting ISIS in Brussels this month.
"We are determined to fight and defeat Daesh," Asiri said, using the Arabic acronym for ISIS. He didn't elaborate on how many troops the kingdom would send.
Saudi Arabia is deeply involved in Yemen's civil war, where it is fighting Iranian-backed Shiite rebels. Saudi Arabia and other Gulf countries have long viewed Iran as a regional menace, and Riyadh and Tehran back opposite sides in the wars in Syria and Yemen.
Asiri's announcement came shortly after Russia said it suspects Turkey of planning a military invasion of Syria.
Ministry spokesman Maj. Gen. Igor Konashenkov said Thursday in a statement that the Russian military has registered "a growing number of signs of hidden preparation of the Turkish armed forces for active actions on the territory of Syria."
He said images of a checkpoint on the Turkish-Syrian border taken in late October and late January show a buildup of transportation infrastructure that could be used for moving in troops, ammunition and weapons.
The announcement came a day after U.N.-led peace talks in Geneva were suspended for three weeks.
The United States blames both Syria's government and Russia for stalling the peace negotiations. Syrian government troops, backed by Russian airstrikes, have increased the pace of attacks on opposition forces in recent days as the talks faltered.
"We believe that the airstrikes alone are not the perfect solution," Asiri said in another interview with the Dubai-based Al-Arabiya TV.