They were rounded up after jihadist commander Abu Musab was imprisoned by Kurdish fighters late on Saturday. The commander was freed on Sunday, various sources said.

"Hundreds of civilian relatives of Kurdish fighters were taken prisoner by the Islamic State of Iraq and the Levant (ISIS)," said the Syrian Observatory for Human Rights.

The civilians were captured hours after the ISIS commander in Tal Abyad, a strategic town on the Turkish border that has been out of regime control since September last year, was detained by Kurdish fighters on Saturday.

The commander, Abu Musab, was freed on Sunday afternoon, said the Observatory.

An activist in Tal Abyad who spoke to AFP by telephone said the town had been besieged by jihadist fighters and snipers after Abu Musab was captured.

"We've lived through a small civil war over the past few hours," said the activist, who asked not to be named for security reasons.

"After Abu Musab was detained, ISIS deployed a large number of snipers and fighters. There have been clashes since last night, and unfortunately there were abuses by all sides," said the activist.

"Many families fled the violence. Tal Abyad is a ghost city. There is now an ethnic-based hatred against the Kurds, though the truth is Kurds and Arabs, Christians and Muslims have always lived here together," he added.

The violence comes just days after fighters loyal to the Committees for the Protection of the Kurdish People (YPG) expelled extremists of the Al-Nusra Front and ISIS from the strategic Kurdish town of Ras al-Ain in Hasake province.

"The Kurdish Front brigade has released the ISIS emir... His release comes after mediation by the armed groups that are seeking the liberation of hundreds of Kurdish civilians," said the Britain-based Observatory.

According to Nasser al-Hajj Mansour, a Kurdish official, the Kurds were imprisoned "on the basis of ethnicity".

"I am not sure how many Kurdish civilians were detained, but what is clear is that there is a will to sow strife," Mansour told AFP.

Jihadist forums meanwhile said Abu Musab was freed by ISIS fighters and not voluntarily by the Kurds.

Mansour warned the violence is likely to escalate.

"We would have wanted things to be more peaceful. But ISIS is forcing an escalation," he said by telephone.

They said he was besieged in a school for 12 hours, and that he was able to flee when his fighters broke the siege.

Kurdish regions have been run by local Kurdish councils since President Bashar al-Assad's forces withdrew from the areas in mid-2012.

Since then, the Kurds have walked a fine line, trying to avoid antagonising either the regime or the rebels.

But this week alone, more than 50 jihadist and Kurdish fighters were killed in fighting in northern Syria, according to a toll released Saturday by the Observatory.

Kurds represent about 15 percent of the Syrian population.