The last pocket of Islamic State territory has fallen, Western-backed Syrian Democratic Forces (SDF) said on Saturday, marking the end of the group's "caliphate" four-and-a-half years after it was declared.

"Total elimination of (the) so-called caliphate", Mustafa Bali, the SDF spokesman, wrote on Twitter. "The military victory against Daesh has been accomplished."

The SDF had Islamic State of Iraq and the Levant (Isil) cornered in a patch of land smaller than one-mile-square in the eastern Syrian village of Baghuz, before finally dealing them defeat.

A small number of Isil fighters, most of them foreign, refused to surrender and fought to the last.