File Photo: Iraqi helicopter crash.
The SDF, which controls most of the northeast, has been a key ally of the US-led coalition fighting jihadists of the Islamic State group in Syria and neighboring Iraq.
Two helicopters carrying SDF counter-terrorism units to Sulaimaniyah, in Iraq's autonomous Kurdistan region, on Wednesday evening "crashed due to bad weather, leading to the death of nine of our fighters," the group said in a statement.
Among the dead was SDF counter-terrorism commander Shervan Kobani -- a cousin of SDF leader Mazloum Abdi.
The SDF said the delegation was on its way to Iraqi Kurdistan "to exchange security and military expertise".
Its statement did not say whether the two aircraft had collided or give any other details of the crash.
Iraqi Kurdish authorities had said Thursday that five people were killed when a single helicopter came down in Dohuk, the most northerly of the provinces that make up the autonomous region.
The regional government's head of foreign media relations, Lawk Ghafuri, said initial investigations suggested "some" of the dead were members of the Kurdistan Workers' Party (PKK), a rebel group that has been fighting the Turkish army since 1984.
Turkey and its Western allies blacklist the PKK as a terrorist group.
But, despite shrill complaints from Ankara, Washington supports the SDF, which is the Kurdish administration's de facto army in the northeast and led the battle that dislodged IS from its last scrap of territory in Syria in 2019.
Ankara regards the dominant faction in the Syrian Kurdish administration, the People's Protection Units (YPG), as an offshoot of the PKK and has mounted repeated armed incursions to force its fighters out of areas near the border.