The Syrian Observatory for Human Rights said on Monday that with US-led air support and smaller Syrian rebel groups, the Kurdish People's Protection Units (YPG) took total control of the Liwa (Brigade) 93 base.

"IS lines of defense were pushed back to the gates of Raqqa," the Observatory said.

The military base lies just 7 km (4 miles) of Ain Issa, a town 50 km (30 miles) north of 'IS' de facto capital Raqqa city from where the terrorist group has proclaimed a caliphate to rule over all Muslims from territory it controls in Syria and Iraq.

"This means that the Islamic State keeps collapsing inside its own stronghold," added Rami Abdel Rahman, head of the Observatory.

Fast-paced

Following the Kurds' deafeat of the jihadists in the border town of Kobani in January, the YPG has rapidly advanced into Raqqa province, defying expectations of the drawn-out battle against 'IS' fighters.

According to Observatory sources, thousands of people had fled from Ain Issa towards Raqqa city in the last two days.

Fears for historical sites

Beyond the Kurdish front in northern Syria, however, there has been little good news for the global campaign against 'IS,' with the jihadists also making major advances in May in both western Iraq and central Syria.

Late on Sunday, the Obsevatory reported that 'IS' had planted landmines and explosives, possibly to destroy ancient structures in the Syrian city of Palmyra.

The move has raised fears after the jihadists destroyed several priceless sculptures and a museum housing treasures of the Babylonian empire in Iraq's Hatra earlier this year.

"It is not known if the purpose is to blow up the ruins or to prevent regime forces from advancing into the town," Rahman told reporters.

ksb/bw (Reuters, AFP)