Free Syrian Army fighters take cover moments after firing a rocket toward government forces in the Idlib province of northern Syria on Friday, September 20.
(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.
"We repeat that we don'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.