Kurdish members of the Syrian Democratic Forces -- made up of Kurds, Syrian Arabs and other groups -- had quit the Middle Euphrates River Valley in February after Turkey sent troops into Syria and launched an operation against the Kurdish People's Protection Units (YPG) militia in its Afrin enclave.

But the SDF last week announced it was relaunching efforts to clear IS from the few areas in Syria where they maintain a presence, including in the eastern oil-rich province of Deir Ezzor near the border with Iraq.

"This week, following an increase in coalition strikes against the final ISIS controlled areas in Syria, the Syrian Democratic Forces resumed offensive operations to clear the final ISIS held territory in eastern Syria," British Army Major General Felix Gedney, a deputy commander of the US-led coalition fighting IS, told Pentagon reporters.

The first phase of operations aims to secure the southeast portion of the Syria-Iraq border in coordination with the Iraqi security forces, Gedney said.

He did not say how many IS fighters remain, though noted that local jihadists are abandoning the fight, causing tension among those remaining.

"Observations from eastern Syria suggest that morale among ISIS fighters is sinking," Gedney said.

"Frictions are mounting between native- and foreign-born ISIS fighters as ISIS's privileged leadership continues to flee the area leaving fighters with dwindling resources and low morale. ISIS fighters continue to surrender rather than face certain death."

He added that more than 400 IS fighters are now in SDF custody.

IS jihadists swept across large parts of Syria and neighboring Iraq in 2014, declaring a so-called "caliphate" in areas they controlled.

But the jihadists have since lost most of that territory to various offensives in both countries.