Beirut (AFP) - Kurdish authorities in northern Syria are arbitrarily detaining critics and alleged Islamic State group sympathisers and denying prisoners fair trials, rights group Amnesty International said on Monday.

In a new report, Amnesty said it had documented several cases of authorities detaining people with little evidence, on occasion as retribution for criticising the ruling Democratic Union Party (PYD).

"The PYD-led autonomous administration cannot use their fight against terrorism as an excuse to violate the rights of individuals under their control," said Amnesty's senior crisis adviser Lama Fakih.

"Instead of trampling all over people's rights in the name of security and counterterrorism, the PYD-led administration should ensure that the rights of detainees are respected," she added.

The rights group said it had interviewed 10 prisoners in two facilities in northeastern Syria.

It acknowledged that prison conditions were "adequate", saying cells were well-equipped and not overcrowded.

But it said some people had been detained for up to a year without charge or trial, and those who went to trial said proceedings were "blatantly unfair".

"They were denied basic rights including the right to defend themselves, to see the evidence against them, and access to a lawyer and their family," the group said.

Some of those detained were Arab residents of areas controlled by Kurdish authorities, who said they were accused of links to IS despite little-to-no evidence.

One said he was detained for nearly a month for having a name similar to a wanted man, and another because of Facebook posts critical of the PYD.

Amnesty said Kurdish authorities had at times "used the counterterrorism law to detain and prosecute Kurdish opposition groups critical of the PYD."

One Kurdish opposition party told Amnesty that 12 of its members had been detained in 2014 and sentenced for "terrorism acts without any substantiated evidence".

The group said Kurdish authorities reported holding some 400 prisoners in total in facilities belonging to the Asayesh security forces throughout regions under their control.

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It was unclear if additional detainees were being held by the Kurdish People's Protection Units (YPG) militia.

Syria's Kurds have established an essentially autonomous zone in parts of the country where they form a majority.

Regime forces have largely withdrawn from the area to focus on other regions, although IS has regularly attacked areas under Kurdish control.

Kurdish forces have served as a key ally of the US-led coalition against IS, calling in air strikes and seizing territory with the help of coalition air support.