Syrian Kurdish-led forces captured a local ISIS commander in eastern Syria as part of an ongoing operation targeting sleeper cells in the city of Raqqa, the US-backed forces announced Thursday. The commander served as the chief of the extremist group’s faction for Raqqa and was among the 68 militants detained in the operation, the Syrian Democratic Forces (SDF) said. The operation started earlier this week, in response to a December attack by ISIS that targeted military and security buildings in Raqqa and killed at least six Syrian Kurdish fighters. A Kurdish commander, Mazloum Abdi, said they had indications of “serious preparations" by ISIS for attacks. The Syrian Observatory for Human Rights said ISIS militants also targeted a military intelligence prison holding some 200 militants in the December attack. The captured ISIS commander was identified as Atallah al-Maythan. Syrian Kurdish forces said he headed the militant group's operations across Raqqa province, and allegedly “confessed to his involvement in planning and leading terrorist acts," extorted money from residents in the area and kept ISIS sleeper cells in contact. Some 5,000 Syrian Kurdish-led fighters are involved in the operation, and have already raided some 80 locations, their spokesperson, Farhad Shami, told The Associated Press. The US-led coalition was providing air support, reconnaissance, and gathering intelligence, Shami added. This is the second recent operation by the US-backed forces in Syria. In late December, the Syrian Kurdish-led fighters targeted ISIS cells in al-Hol and Tal Hamis, following a surge in militant attacks. The US Central Command said that 215 militants from ISIS were arrested last year and 466 were killed in Syria. There are roughly 900 US troops in Syria.