An Arab-Kurdish alliance advanced Wednesday (December 21st) to several kilometres from the largest prison held by the "Islamic State of Iraq and the Levant" (ISIL) in Syria's northern province of al-Raqa, AFP reported.

Members of the Syrian Democratic Forces (SDF) were eight kilometres from the jail near the country's largest dam at Tabqa on the Euphrates River, the Syrian Observatory for Human Rights said.

On December 10th, the SDF announced "phase two" of its campaign against ISIL's de facto Syrian capital of al-Raqa, some 50 kilometres east of Tabqa in the same province.

The alliance has since captured dozens of villages and hamlets, the Observatory said, after taking 700 square kilometres from the extremists in a first phase of the assault from November 5th .

Western hostages of ISIL are believed to have been held at the jail near the town of Tabqa, where top ISIL leaders live, the Observatory said.

Kurdish officials told AFP on Wednesday that many of the jihadists had fled before the SDF advance.

ISIL fighters "have no more heavy weapons -- they are weakened and flee without resisting", said Moslem Sheikh Nouh, a commander with the alliance.