UPDATE 1-US-backed Syrian fighters claim to have captured IS leader's assistant

U.S.-backed fighters in Syria have captured a senior Islamic State leader who served as an assistant to the group's self-declared "caliph" Abu Bakr al-Baghdadi, they said on Friday.

The Syrian Democratic Forces (SDF), a Kurdish-led militia alliance that holds the quarter of Syria east of the Euphrates, detained Osama al-Awaid last week, it said in a statement.

SDF fighters have been attempting to take Islamic State's last Syrian pocket of territory on the banks of the Euphrates near Iraq for weeks, in an offensive backed by U.S.-led air strikes.

They captured Awaid in a special operation in a village in eastern Syria, the statement said, adding that he had been a senior security official for Islamic State in the country.

More than a year after Islamic State's physical "caliphate" collapsed in both Syria and Iraq under military attack, Baghdadi's whereabouts remain a mystery.

However, despite its loss of territory, the group has been able to launch guerrilla attacks in areas controlled by both the Syrian and Iraqi governments.