The human rights organisation Amnesty International has called on Syria to release Kurds detained during last month's violent riots.
Trouble started after a football match and quickly spread
"Hundreds" of Kurds are being held at unknown locations, incommunicado and without charge, Amnesty said.
It called for an independent inquiry into the riots which left 25 dead and injured hundreds in five days.
The unrest was sparked by a soccer match brawl on 12 March, between fans of teams supported by Arabs and Kurds.
It spread quickly from the northern town of Qameshli, near the border with Turkey and Iraq,
to other towns.
The Syrian government has not revealed how many people were detained following clashes between demonstrators and troops.
Kurdish leaders said last month that hundreds of people had been released from custody, but that hundreds of others were still being held.
In its statement on Tuesday, Amnesty International said: "The incommunicado detention at unknown locations of many hundreds of Syrian Kurds is of serious concern, not least as it puts detainees at greater risk of torture or
ill-treatment," the statement said.
"Unless they are to be charged with recognisably criminal
offences and brought to trial without undue delay, they should be released immediately,"
The group also urged the Syrian authorities to establish how friction at a soccer match escalated into such violence.
Syria has about two million Kurds among its population of 17 million.