Brigadier General Frederic Parisot, who heads civil-military operations for the US-led coalition fighting IS in Syria and Iraq, told Pentagon reporters the jihadists now hold an area of about 30 by 10 kilometers (18 by six miles).

This terrain is located in eastern Deir Ezzor province, and includes the town of Hajin, about 50 kilometers (30 miles) from Iraq's border.

Parisot said the fight against IS would last "for at least two or three months" and noted that a few hundred jihadists remain.

"We don't know how many. But we want to make sure that we kill all of them," he said.

Pentagon officials added that since April 24, the coalition has killed six IS leaders and planners in strikes.

IS declared a cross-border "caliphate" in Syria and Iraq in 2014, seizing a third of Iraq and vast chunks of northern Syria during a sweeping offensive.

The jihadists have since lost almost all the ground to separate counter-offensives by Syrian and Iraqi forces as well as US-led operations.

In December the Iraqi government declared victory over IS but the military has continued regular operations targeting mostly desert areas along the porous Syrian border.