WASHINGTON (Reuters) - More than 400 U.S. Marines and their artillery are leaving Syria after helping to capture the city of Raqqa from Islamic State, the U.S.-led coalition fighting the militant group said on Thursday.  Last month, U.S.-backed militias declared victory in Raqqa, Islamic State s former headquarters in Syria, after months of fighting with the help of the U.S.-led coalition.  With the city liberated and ISIS on the run, the unit has been ordered home. Its replacements have been called off,  the coalition said in a statement, using an acronym for Islamic State.  We re drawing down combat forces where it makes sense, but still continuing our efforts to help Syrian and Iraqi partners maintain security,  Brigadier General Jonathan Braga, the director of operations for the coalition, said in the statement.  Our remaining forces will continue to work by, with, and through partner forces to defeat remaining ISIS, prevent a re-emergence of ISIS, and set conditions for international governments and NGOs to help local citizens recover from the horrors of ISIS  short-lived rule,  he said. Officially, the Pentagon says there are 503 troops in Syria. However, as of last week, U.S. officials said there were closer to 2,000 U.S. troops in the country. The latest announcement would reduce that number. As that campaign against Islamic State winds down, it is unclear how many, if any, U.S. troops will remain in Syria. Most of them are special operations forces, working to train and advise local partner forces, including providing artillery support against Islamic State militants. Separately, Russia said it was already preparing to withdraw its military contingent from Syria, Russian news agencies reported. 