(Adds quotes, comments on attack on prison, details, context)
BEIRUT, Oct 10 (Reuters) - A senior Kurdish official warned on Thursday that Islamic State militants could break out of prisons in northeast Syria as fighting intensifies between the Kurdish-led Syrian Democratic Forces (SDF) and Turkey.
Badran Jia Kurd told Reuters the number of security forces guarding the IS detainees would have to be reduced as the Turkish forces step up an offensive that got underway on Wednesday.
"This attack will definitely reduce and weaken the guarding system for those Daesh militants in the prisons," he said, using the Arabic acronym for Islamic State.
"This could lead to their escape or to behaviors that may get out of the control of the security forces," added Jia Kurd, adviser to the Kurdish-led authority running much of north and east Syria.
"The number of forces guarding the prisons is reduced the more the battles intensify. This poses a grave danger."
Turkey launched an offensive on Wednesday against Kurdish fighters in northeast Syria, after U.S. forces withdrew from part of the border region, opening up a dangerous new phase in Syria's more than eight-year-old conflict.
The SDF holds thousands of Islamic State fighters in prisons and tens of thousands of their relatives in camps, many of them foreigners. With the Kurdish YPG militia at its forefront, the SDF defeated jihadists in swathes of north and east Syria with U.S. help including ground forces and air power.
The border strip which U.S. forces vacated this week, and which is believed to be the focus of Ankara's offensive, is a nearly 100-km (62-mile) stretch between the Syrian towns of Tel Abyad and Ras al-Ain. It does not include prisons.
The Kurdish-led authority accused Turkey of bombing a prison holding jihadists of more than 60 nationalities. The shelling on Wednesday night targeted part of Chirkin prison in the city of Qamishli, it said.
Turkey has yet to respond to the accusation. (Reporting by Ellen Francis Editing by Peter Graff)