ISIS militants in northern Syria launched a failed attack targeting a prison holding fellow extremists in a clash that killed six Kurdish fighters, local authorities and a war monitor said. The Kurdish fighters were killed in the assault on a security complex near a prison with ISIS inmates in Raqqa, the group's former de facto capital in Syria, said Farhad Shami, spokesperson for the Kurdish-led Syrian Democratic Forces. But "ISIS failed to attack the prison because our forces thwarted their assault", he said. He confirmed the six deaths to AFP, adding that security forces killed a militant wearing an explosive belt and apprehended his accomplice. Britain-based war monitor, the Syrian Observatory for Human Rights, had earlier reported two extremists killed. The failed assault targeted a Kurdish security complex, which includes a military intelligence prison housing militants, the monitor said. "The militants were targeting the military intelligence prison" housing hundreds of extremists, including 200 high-level fighters, said Rami Abdel Rahman, who heads the Observatory. SDF commander Mazloum Abdi said in a statement that "terrorist cells" had attacked "security and military buildings in the city". "The information we have from Raqqa indicates that ISIS cells are preparing dangerous plots," he added. Kurdish-led authorities announced a state of emergency in Raqqa and have put the city on lockdown until further notice, as security forces hunt down militants at large. Security forces are still searching the area to arrest members of the cell, spokesperson Shami said. ISIS claimed responsibility for the attack, saying two of its fighters had launched it, one of whom had escaped. The group said the attack came to avenge "Muslim prisoners" and female relatives of militants living in the Kurdish-administered Al-Hol camp. Al-Hol, home to more than 50,000 people, is the largest camp for displaced people who fled after the SDF led the battle that dislodged ISIS fighters from the last scraps of their Syrian territory in 2019. Among Al-Hol's detainees are more than 10,000 foreigners from dozens of countries. The overcrowded camp is also home to displaced Syrians, and Iraqi refugees. This is the most significant militant attack on a prison since ISIS fighters launched their biggest assault in years in January, when they attacked the Ghwayran prison in the Kurdish-controlled city of Hasakeh. Hundreds were killed in the assault that lasted for a week and aimed to free fellow militants.