Police in Syria's beleaguered self-ruled Kurdish areas have rounded up hundreds of young men to press them into military service recently made compulsory, activists and Kurdish officials say.

The unprecedented move, criticized by human rights groups, underlines the desperation of Syria's Kurds as they fight off an onslaught by the Islamic State extremist group in Kobani, a town near the Turkish border.

More than 550 people have been killed since the group began its offensive on the Kurdish town in mid-September, and the fighting has forced more than 200,000 people to flee into Turkey.

The extremists have now seized a third of Kobani, closing in on positions held by the main Kurdish militia — the People's Protection Units, or YPG.

In majority Kurdish areas east of Kobani, Kurdish police forces known as Asayish began Saturday rounding up scores of people, according to the Britain-based Syrian Observatory for Human Rights, handing them over to defense authorities who are holding them in schools in preparation for training.

Mustafa Osso, a senior member of the National Kurdish Council, said police set up checkpoints and raided homes, adding that they had so far detained some 700 men under the age of 30.

"This has a direct link to what is happening in Kobani," Osso said, alluding to Kurdish fears of being overrun by the Islamic State group militants.

Deadly attacks have taken place in predominantly Kurdish areas outside Kobani, although there has not been sustained fighting.

On Oct. 6, three suicide bombings targeting security forces in the Hassakeh area killed some 30 people, said Kurdish official Nawaf Khalil. Activists have blamed the Islamic State group for the attacks.

Osso opposes rounding up and detaining the young men, saying that historically Kurdish fighting forces have been based on volunteers, not conscription.

"This will prompt young men to immigrate," he warned, speaking by telephone from Turkey.

The Kurds, long ostracized in Syria, took advantage of the chaos of the country's civil war to carve out a semi-autonomous territory in the north.

In November, they declared their own civil administration in areas under their control, dividing it into the regions named Afrin, Kobani and Jazeera.

In mid-July, Kurdish authorities in Jazeera imposed a compulsory six-month military service on adult males in order to bolster forces fending off Islamic extremists. Although the area is majority Kurdish, Arab, Armenian and Christian minorities living in the area are also required to do the service.

Ekrem Hasso, an official at the Jazeera administration, said that the administration is "committed to implementing its laws." He acknowledged however that Asayish police forces may have shown "some bad behavior" at some checkpoints. He did not elaborate.

Seven Syrian human rights groups condemned the detentions, calling them "arbitrary arrests." In a statement, they said that besides Kurds, a variety of groups were among those detained, including Syriacs, Assyrians, Chaldians, Arabs and Armenians.

The Observatory said the young men will be referred to defense officials who will conduct medical tests and interview them for military service.

"We call upon the civil administration to immediately release the detainees without conditions and to abstain from such practices," they said in an emailed statement.

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Follow Bassem Mroue on Twitter at www.twitter.com/bmroue .