QAMISHLI, northeast Syria (Reuters) - Kurdish-led authorities controlling northeastern Syria will not be able to hold foreign Islamic State fighters indefinitely, and their home countries should take them back, a senior official there said on Thursday.

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Abdulkarim Omar, joint head of foreign relations in the Kurdish-led area, told journalists its administration was holding around 500 foreign fighters and 500 family members from around 40 countries, following last year’s defeat of Islamic State in nearly all territory it once held in Syria and Iraq.

“For us it is a very large number because these Daeshis are dangerous and they committed massacres, and their presence in our detention is an opportunity for the international community to put them on trial,” Omar said, using an Arabic acronym for Islamic State members.

Backed by the United States and its allies, the Kurdish-led Syrian Democratic Forces (SDF) militia alliance captured swathes of northern and eastern Syria from Islamic State over the past two years, including the jihadists’ one-time capital Raqqa.

The SDF is now fighting to take the last few villages Islamic State holds along the Euphrates River in Syria, close to the border with Iraq, and has detained more foreign fighters, Omar said.

Omar said the administration in the area lacked the resources to properly rehabilitate so many prisoners. It would put Syrians on trial, but not foreigners, and it would not execute anyone as it did not impose the death penalty.

“We will try on the path of dialogue... to hand them over to their countries, but if our hope is cut, we will have other options,” he said. He declined to explain what he meant by “other options” apart from no longer detaining the prisoners.

Omar was speaking at a news conference in Qamishli near the Turkish border to announce that Sudan was taking back a captured Sudanese woman who had joined Islamic State.

She was one of only 50 or 60 people taken back by their countries so far, including women and children, Omar said. He said Russia and Indonesia had each taken back families.

The continued presence of foreign fighters in an unstable part of the world posed a danger to the whole international community, because they might take advantage of any new period of chaos to escape, Omar said.

“We alone cannot bear this burden,” he said. “This problem is no less dangerous than that of the Daesh state.”