BEIRUT (Reuters) - The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias, said on Friday it had taken the last districts in the old city of Raqqa from Islamic State, but the U.S.-led coalition which backs it could not confirm the report.  We declare to our people the liberation of the old city of Raqqa,  the SDF said in a statement.  The SDF has been battling to capture the former de facto capital of Islamic State s self-declared caliphate since June with backing from U.S.-led jets and special forces.  The walled old city lies in the heart of Raqqa but the jihadist group still holds important districts in the west of the city. The SDF said it now held 65 percent of Raqqa in total. A war monitor, the Britain-based Syrian Observatory for Human Rights, said it was not true that the SDF had fully captured the old city, but added that it did hold more than 90 percent of that area.  The coalition said it was not yet able to confirm the news.  We have not received confirmation of that through our channels,  coalition spokesman Colonel Ryan Dillon said.  