ISTANBUL/WASHINGTON, May 7 (Reuters) - The U.S. military has started training Syrian fighters to combat Islamic State militants, U.S. and Middle East sources told Reuters on Thursday, speaking on condition of anonymity, adding the program had begun in Jordan and would soon launch in Turkey.

The U.S. plan to train and arm a force that is expected to eventually total more than 15,000 troops is a major test of President Barack Obama's strategy in Syria, which critics say is too limited to steer events.

The Pentagon declined comment. (Reporting by Phil Stewart in Washington and Dasha Afanasieva in Istanbul; Editing by Will Dunham)