BAGHDAD, Oct 11 (Reuters) - Iraqi security forces have captured a senior member of the Islamic State group who was a deputy to slain leader Abu Bakr al-Baghdadi and oversaw its finances, Iraqi Prime Minister Mustafa Al-Kadhimi said on Monday. Sami Jasim was detained in "a complex external operation" by Iraqi intelligence services, Kadhimi wrote on Twitter, without giving further details or saying where he had been captured. Jasim, an Iraqi national, is one of Islamic State"s core leaders who may offer valuable information on the group"s operations, said Hassan Hassan, an expert on the group. He is only the second senior IS leader to be taken alive, he said. Baghdadi, who declared himself leader of a cross-border "caliphate" in Syria and Iraq in 2014, was killed in an operation by U.S. special forces in northwestern Syria in 2019. While Islamic State was driven out of most of the territory it once held in Syria and Iraq several years ago, Western military officials estimate that it still has at least 10,000 fighters across the two countries, typically in remote areas. “While we are not commenting on any specific operation, we applaud our brave Iraqi partners as they regularly lead and conduct destructive blows to the remnants of (IS)," lieutenant colonel Joel Harper, spokesman for the U.S.-led coalition, said in a response to a question from Reuters about Jasim"s capture. The U.S.-led coalition is working with Iraqi security forces and the Kurdish-led Syrian Democratic Forces in Syria against remnants of the group. In Iraq, IS militants still wage regular attacks on police, army and Iraqi state paramilitary units, killing dozens of police and fighters in the past year. Hassan, author of a book on Islamic State and editor in chief of New Lines Magazine, said Jasim is a member of Islamic State"s top leadership council, the delegated committee, which has between half a dozen and a dozen members, and is a close aide of the group"s leader, Abu Ibrahim al-Hashemi al-Quraishi. His role had expanded from overseer of the group"s finances to coordinating activities between Iraq and Syria, Hassan said. "He is involved in the day to day operations of ISIS in Syria and Iraq, so strategically and tactically, this is a significant capture for the Iraqis," he told Reuters. "Usually, (IS leaders) blow themselves up or fight to the end." According to the website of the U.S. Department of State"s counter-terrorism rewards programme, Rewards for Justice, Jasim has been "instrumental in managing finances for ISIS"s terrorist operations". "While serving as ISIS deputy in southern Mosul in 2014, he reportedly served as the equivalent of ISIS’s finance minister, supervising the group’s revenue-generating operations from illicit sales of oil, gas, antiquities, and minerals," it says.