"Daesh (IS) early this morning took control of the Al-Walid post on the border between Iraq and Syria after the withdrawal of the army and the Iraqi border police," a police colonel said.
The jihadists had seized the Syrian side, known as Al-Tanaf, three days earlier, leaving Iraqi forces guarding the remote outpost in Anbar province very vulnerable.
The police colonel said the government forces at Al-Walid temporarily pulled back to the nearby Trebil border crossing with Jordan.
The head of Anbar's border commission confirmed that government forces had pulled out of Al-Walid.
"There was no military support for the security forces and there weren't enough of them to protect the crossing," Suad Jassem said.
"Daesh now controls both sides of both crossings," she said.