The Kurdish Democratic Union Party (PYD), which has controlled large swathes of Hasakeh province in northeastern Syria since government troops withdrew from Kurdish-majority areas last year, said one of its fighters was killed in an ambush by a rival armed group.

Dozens of demonstrators were also wounded when PYD fighters opened fire on Thursday on a protest against the militia's detention last week of a group of activists opposed to its grip on the town, the Britain-based Syrian Observatory for Human Rights said.

"At about 7:00 pm (1600 GMT) on Thursday, hundreds of residents of Amuda... gathered in the streets to demand the release" of three detainees from the PYD's prisons, said citizen journalist Havidar.

As protesters chanted anti-PYD slogans, "the Asaish (the PYD's security police) opened fire... on the protesters," Havidar said.

Amateur video distributed by activists showed fighters on a white pick-up truck firing live rounds as cries were heard from the crowd.

The PYD said its fighters had been ambushed by a rival armed group in Amuda, on the border with Turkey.

"A PYD fighter has been killed at the hands of mercenaries from Amuda... Two others have been injured," it said in a statement.

But independent Kurdish journalist and activist Massoud Akko said he did not believe any protesters were armed.

"The protest was peaceful... There is no excuse for the use of weapons there," Akko told AFP via the Internet.

Amuda-based activist Neeshan Malle said that the PYD, the largest of the armed groups operating in Kurdish areas, had imposed a curfew.

"No one is allowed on the streets, and there are snipers everywhere. All the shops are closed," Malle told AFP via the Internet.

"Only one bakery is open in the town. But we think the crisis might be solved today," Malle added.

The Kurdish National Council, another key group, issued a statement denouncing "the shameful incident".

It said it was "convinced" the Amuda crisis "is a result of the divisions among Kurds", and that "it requires efforts by us all... not to be dragged to escalation".

Syria's main opposition National Coalition, meanwhile, also denounced the violence, calling in a statement on "all sides in Amuda... to exercise restraint".

Kurds form significant minorities in Iraq, Iran and Turkey as well as in Syria, where they make up around 15 percent of the population, living mainly in the northeast and northwest.

Long marginalised by successive Arab nationalist regimes in Damascus, they have taken advantage of the uprising that broke out in March 2011 to assert their language and culture in Kurdish-majority areas.

The PYD champions a similar leftist, Kurdish nationalist platform to the PKK, the group which struck a ceasefire deal with Ankara this year after a bloody 29-year insurgency for self-rule in southeastern Turkey.