BEIRUT -- Kurdish-led fighters advanced slowly Saturday under the cover of U.S.-led coalition air power in Syria's northeast, after intense clashes with Islamic State group militants took place around a prison where thousands of extremists were held, officials said.
Fighting broke out late Thursday when Islamic State unleashed its biggest attack in Syria since the fall of its "caliphate" three years ago. More than 100 militants assaulted the main prison holding suspected extremists in the northeastern city of Hassakeh, sparking a battle with U.S.-backed Kurdish fighters that has so far left dozens dead.
The Kurdish-led Syrian Democratic Forces lost 17 fighters killed and 23 wounded since the fighting began, spokesman Farhad Shami tweeted Saturday. Dozens of Islamic State gunmen were also killed.
Despite its defeat in Syria nearly three years ago, the group's sleeper cells have carried out deadly attacks against the Syrian Defense Forces as well as government forces on the west bank of the Tigris River in eastern Syria.
The U.S.-backed Kurdish-led Syrian Democratic Forces have brought more reinforcements into Hassakeh in an attempt to regain control of areas taken by Islamic State, residents said. More civilians fled the areas of fighting as sounds of explosions echoed in the city and black smoke billowed from the Gweiran Prison area on the southern edge of Hassakeh.
Hassakeh Gov. Ghassan Khalil told Syrian state media that some 4,000 civilians have fled to areas controlled by Syrian government forces in the city and its suburbs. He told state TV that authorities set up three shelters for the displaced and mosques were also asked to open their doors for those who were forced to leave their homes.
Gweiran Prison, with more than 3,000 inmates, is the largest of around a dozen facilities run by U.S.-backed Syrian Kurdish forces holding suspected Islamic State fighters.
"The battles are taking place on the edge of the prison," Syrian Defense Forces spokesman Siamand Ali told The Associated Press, adding that most of the prison is under their control apart from a small part that is held by rioting prisoners. He added that fighting is also ongoing in the nearby Zuhour neighborhood, where Islamic State fighters were holed up.
Ali said Syrian Defense Forces fighters and U.S.-led coalition aircraft targeted a technical academy building where dozens of "Daesh terrorists took positions." Ali, who used the Arabic acronym for the Islamic State, said Syrian Defense Forces fighters are advancing slowly in order to protect the lives of civilians as Islamic State gunmen are holed up in alleys and in residential homes.
Ali said some 45 Islamic State gunmen were killed in the fighting and dozens of prisoners who fled were recaptured.
The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said that since Thursday night, 89 people have been killed, including 56 Islamic State gunmen, 28 Kurdish fighters and five civilians.
Pentagon press secretary John Kirby said Friday the U.S. used airstrikes to support the Syrian Defense Forces.
On Friday, a group spokesman said they recaptured 104 militants who escaped from the prison. But he said the total number who had broken out was not determined.