( CNN ) -- Infighting between Syrian rebels threatened a hospital in the northern city of Aleppo on Sunday , a source said , forcing doctors to hide patients as clashes flared between Islamists and members of the Free Syrian Army . `` Doctors are working neutrally , treating both sides , '' said the source in northern Syria , who asked to remain anonymous for fear of reprisal attacks . As doctors treat the wounded , fighters from the FSA and the al Qaeda-linked Islamic State of Iraq and Syria , or ISIS , are fighting outside the hospital and plotting to attack each other inside , the source said . The clashes come after the FSA gave ISIS members a 24-hour ultimatum to surrender and leave the country and arrested 200 members of the al Qaeda-linked group . Spain arrests suspected militant returning from Syria . `` We repeat that we do n't want any foreign fighters in Syria , '' FSA spokesman Loauy Mokdad told CNN on Saturday . `` We do not want any terrorist groups in Syria . We will not allow them to make bases in Syria . '' For months , the groups maintained an uneasy alliance as they fought to topple Syrian President Bashar al-Assad 's government . But now , infighting within the rebel groups has threatened to tip the balance among rebel forces toward militant groups and away from more secular brigades . The FSA has accused ISIS of killing FSA officers and committing crimes against civilians . `` After six months attempting to hijack our revolution and after they controlled some liberated areas and after they start to force our people to act like Islamic state , we gave so many warnings to them , that 's something not acceptable for us , '' Mokdad said . ISIS issued its own ultimatum on Saturday , giving the FSA 24 hours to free their prisoners , or they will leave their joint posts with the FSA in Aleppo . Meanwhile , the source said , time is running out at hospitals in the violence-ravaged area , where diesel fuel needed for generators and ambulances is in short supply and could run out within days . Doctors , the source said , face danger from all sides . While rebels clash , the source described how government forces are closing in on three sides of the city . `` The medical staff in Aleppo will be targeted by ISIS if they know that they are treating FSA patients , by FSA if they know that they are treating ISIS patients , and by ( the ) regime 's airstrikes if they know that they are treating both , '' the source said .
Soldiers from the Free Syrian Army are fighting with al Qaeda - linked fighters . Both are trying to overthrow al - Assad 's government ; now , they 're fighting each other . ` ` forces are fighting neutrally , on both sides , '' source says .