WASHINGTON - US President Donald Trump on Saturday hailed the defeat of Islamic State fighters in their self-proclaimed capital of Raqqa as "a critical breakthrough" in a worldwide campaign against the militants.On Friday, the Kurdish-dominated Syrian Democratic Forces (SDF) formally announced Raqqa's liberation from Islamic State after four months of battles and said the city would be part of a decentralized federal Syria. "Together, our forces have liberated the entire city from ISIS control," Trump said in a statement."The defeat of ISIS in Raqqa represents a critical breakthrough in our worldwide campaign to defeat ISIS and its wicked ideology. With the liberation of ISIS’s capital and the vast majority of its territory, the end of the ISIS caliphate is in sight."Trump said the US campaign against Islamic State, which was launched by his predecessor Barack Obama, would soon enter a new phase, in which the United States would "support local security forces, de-escalate violence across Syria, and advance the conditions for lasting peace, so that the terrorists cannot return to threaten our collective security again.""Together, with our allies and partners, we will support diplomatic negotiations that end the violence, allow refugees to return safely home, and yield a political transition that honors the will of the Syrian people," he said.Trump's statement made no mention of the future of Syrian President Bashar al-Assad. He also did not spell out how the United States would support local security forces.A White House spokesman said US policy towards Assad "remains the same." US officials have said Assad has no future governing Syria and US envoy to the United Nations, Nikki Haley, said last month a stable Syria was not possible while he remained in place.The fight against Islamic State has taken place amid a wider, multi-sided civil war between Assad's government, which is backed by Iran and Russia, and an array of rebel groups supported by other powers, including the United States.Experts believe the defeat of ISIS at Raqqa may only be the start of a wider struggle by the United States to contain any insurgency launched by the militant group and to stabilize the region, as Washington grapples with defining a comprehensive strategy in Syria.On Friday, French President Emmanuel Macron said France's military would continue its fight against Islamic State in Syria, but that the fall of the militant group's bastion in Raqqa needed to lead to an inclusive political system to restore stability.