BEIRUT (Reuters) - Raqqa will be part of a decentralized federal Syria now the city has been freed from Islamic State, the U.S.-backed militias that captured it said on Friday, tying its political future to Kurdish-led autonomy plans for northern Syria. The Kurdish-dominated Syrian Democratic Forces (SDF) said the people of the majority Arab city and surrounding province would decide their own future  within the framework of a decentralized, federal, democratic Syria . In a declaration formally announcing Raqqa s liberation from Islamic State after four months of battles, the SDF pledged  to protect the frontiers of the province against all external threats , and to hand control to a civil council from the city. The U.S.-backed SDF, which is led by Kurdish militia fighters and also includes Arabs, captured the Syrian de-facto capital of Islamic State s self-proclaimed caliphate this week in a decisive defeat for the group that had ruled over millions of people from central Syria to northern Iraq. In Syria, the fight against Islamic State has taken place amid a wider, multi-sided civil war between the government of President Bashar al-Assad, who is backed by Iran and Russia, and an array of rebel groups supported by other powers. Kurdish-led authorities in other parts of the north say they want a federal system that would allow regions to rule themselves without central control by the center. They have been moving ahead with plans set it up, despite discouragement from their U.S. allies and strong opposition from both Turkey and the Syrian government in Damascus.  Turkey views the rise of Kurdish power in Syria as a threat to its national security, viewing Kurdish groups in northern Syria as an extension of the Kurdistan Workers  Party (PKK), which is fighting a three-decade insurgency in Turkey. Turkey said on Friday that a huge banner of jailed PKK founder Abdullah Ocalan unfurled in central Raqqa by the U.S.-backed Kurdish forces was proof that Washington had been working with terrorists. The Kurdish-led autonomy plans in Syria have advanced as diplomacy has failed to make any progress towards a political solution to the six-year-long war. Assad, who is fast regaining territory in Syria s most populous eastern regions with Iranian and Russian military support, has repeatedly said the state will recover all Syria. Asked about the SDF statement, Ali Haidar, the Syrian government minister responsible for national reconciliation, said Raqqa s future could only be discussed  as part of the final political structure of the Syrian state . The SDF, dominated by the Kurdish YPG militia, has established control over swathes of northern Syria in operations backed by the U.S.-led coalition against Islamic State, mostly avoiding conflict with Russian-backed pro-government forces. Spokesman Talal Silo reaffirmed the SDF s plan to hand control of Raqqa to a civil council formed of people from the city, and to hand over internal security to a police force drawn from local recruits. He called Raqqa s liberation  the final chapter  of a struggle that began against Islamic State from the town of Kobani some three years ago, though the SDF is still fighting the jihadist group in Deir al-Zor province. The United States has said it will take the lead in helping to clear rubble and restore basic services to the city. With Islamic State s self-declared  caliphate  crumbling in its last remaining Syrian strongholds, Assad s Russian allies said on Thursday the fight with the jihadist group would soon be over. President Vladimir Putin expressed hope a new peace conference could be convened, though the Kremlin said on Friday it was too early to discuss the timing or venue for the proposed  congress of the peoples of Syria . 