Doctors Without Borders says the delivery of humanitarian aid to half a million newly displaced civilians in northeastern Syria is threatened because of fighting between Iraqi forces and Kurdish fighters near the border.

The medical charity, also known as MSF, said Sunday it is "extremely concerned" because the fighting threatens its only cross-border supply routes between the two countries.

The Fishkhabur crossing is the only gateway for residents, aid workers and journalists to enter northeastern Syria. A nearby crossing, used for commercial traffic, was closed on Thursday due to fighting in Iraq, and has yet to reopen.

Scattered clashes have erupted in recent weeks as Iraqi forces have retaken disputed territory from the Kurds, part of a crisis sparked by last month's Kurdish vote for independence.