The United States has already spent more than $3 billion since it joined the campaign in August last year to get rid of the Islamic State jihadist group which has entrenched itself in parts of Iraq and Syria, the US Department of Defense said.

The total cost of the US military's activities against ISIS under Operation Inherent Resolve has risen to $3.21 billion since operations started in Aug. 8 last year, the Pentagon announced in a statement.

This means that as of July 15 this year, an average of $9.4 million is being spent daily by the US in the operation which seeks to eliminate the terrorist group and the threat it poses to Iraq, Syria, and the international community.

The average tab ballooned to an average of $9.9 million per day when the US started to operate both in Iraq and Syria. This is considerably higher from the average cost of $5.6 million daily in August and September last year, when the US was only fighting the jihadist organisation in Iraq, the Pentagon said.

Most of the expenses go to the Air Force for its daily flight operations, which take a lion's share of 53 percent or $1.70 billion of the total costs, a CNN report said.

Munitions and operational support follow, taking 23 percent of the total cost, respectively.

Operation Inherent Resolve has so far damaged or destroyed a total of 7,655 targets as of June 22, Pentagon officials said.

Of this, 2,702 have been labelled "other targets" while 2,045 are buildings and 1,859 are fighting positions.

The US military also targeted tanks, high mobility multi-purpose wheeled vehicles or HMMWV, staging areas, and oil infrastructure, according to Pentagon sources.

The US Department of Defence said "frontlines in much of northern and central Iraq have been pushed back" since August.

"ISIL can no longer operate freely in roughly 25 to 30 percent of populated areas of Iraqi territory where it once could," said the Pentagon statement, using another name for ISIS.

The group's controlled territory in Syria "remains largely unchanged, with its gains in As Sywayda, Damascus countryside, and Homs provinces offset by losses in Halab and Al Hasakah Provinces," the Pentagon said.