The U.S. military has started training Syrian opposition fighters in Turkey to combat self-proclaimed Islamic State in Iraq and Syria, an expected expansion of a programme that first launched in Jordan weeks ago, a U.S. official told Reuters on Thursday.The official, who spoke on condition of anonymity, did not offer details on the size of the first group of recruits undergoing training in Turkey or the specific start date.The Pentagon declined to comment.President Barack Obama's administration says the programme aims only to target ISIS forces, not troops loyal to Syrian President Bashar al-Assad. But critics, including in the U.S. Congress, say that theoretical limitation is unlikely to withstand the realities of Syria's messy civil war.The war in Syria has killed more than 220,000 people and left a third of the population homeless.The United States hopes the long-awaited programme will train just over 5,000 Syrian fighters a year, giving the U.S. military partners on the ground to combat ISIS.So far, the U.S. role in Syria has been largely limited to air strikes, although American special operations forces killed a senior ISIS leader in a raid there this month.All of the U.S. military training of Syrian opposition fighters is taking place outside of the country. Beyond Jordan and Turkey, Saudi Arabia and Qatar have also offered to host training sites.