Beirut - US-backed Syrian forces liberated the city of Raqqa from Islamic State militants on Tuesday, a senior commander for the force said, adding that clearing operations were underway to remove land mines left behind and search for the extremist group's sleeper cells.Brig Gen Talal Sillo said that there were no longer clashes in the city, which had served as the extremist group's headquarters and self-proclaimed capital of their so-called "caliphate" for more than three years.Lost territoryRaqqa is still full of land mines, Sillo added, but fighters of the Syrian Democratic Forces were now in control of the former "capital of terrorism".The head of a police force affiliated with the SDF was killed in a land mine explosion in the city, on Monday, he said.Losing Raqqa is a huge blow for ISIS, which has steadily lost territory in Iraq and Syria, including Iraq's second largest city of Mosul few months ago. The group declared the city on the banks of the Euphrates River, which it seized from other Syrian rebels in early 2014, to be the capital of its self-styled "caliphate," transforming the one vibrant metropolis into the epicentre of its brutal rule where opponents were beheaded and terror plots were planned.Brutal ruleDozens of militants who refused to surrender had made their last stand in the city's stadium, which had become notorious as a prison and dungeons for the group.The Kurdish-led SDF forces earlier captured the city hospital, the other last remaining ISIS holdout in Raqqa. The facility had doubled as a hospital and an ISIS command centre.Its capture left ISIS militants cornered in and around the notorious stadium, which they had turned into a huge prison where they incarcerated anyone who opposed their brutal rule. After Sillo's statement, it was not immediately clear if the ISIS militants were still inside the stadium.The battle for Raqqa began in June and has dragged for weeks as the SDF fighters faced stiff resistance from the militants. In the campaign, the city suffered major devastation, leaving most of its buildings leveled and in ruins.