(Adds comments from Brennan, details)
WASHINGTON, June 16 (Reuters) - The director of the U.S. Central Intelligence Agency, John Brennan, said on Thursday there were tens of thousands of Islamic State fighters around the world, more than al Qaeda at its height.
He also told a Senate Intelligence Committee hearing that the agency was concerned about the growth of Libya as a base of operations for Islamic State militants, who had 5,000-8,000 fighters there, although the group's fighters in Iraq and Syria had dropped to 18,000 to 22,000 from 19,000 to 25,000.
"I am concerned about the growth of Libya as another area that could serve as the basis for ISIL to carry out attacks inside of Europe... that is very concerning," Brennan said, using an alternative acronym for the Islamic State militant group.
Questioned about the broader crisis, Brennan told lawmakers he believed the government of Syrian President Bashar al-Assad had been strengthened with Russia's support.
"A year ago, (Assad) was on his back foot as the opposition forces were carrying out operations that were really degrading the Syrian military. He is in a stronger position than he was in June of last year" as a result of Russian support, Brennan said.
(Reporting by Patricia Zengerle and Jonathan Landay; Editing by Chizu Nomiyama and Bernadette Baum) ((firstname.lastname@example.org; 001-202-898-8390; Reuters Messaging: email@example.com))