WASHINGTON (AFP) - US war planes struck more than two dozen times in Syria and Iraq on Friday and Saturday, hitting Islamic State (IS) militants and oil infrastructure they control, the American military said.

Of 15 air strikes in Syria, 12 were aimed at "degrading and destroying their oil producing, collecting, storage and transportation infrastructure," the US Central Command said in a statement.

The militants control a swath of territory straddling northwestern Iraq and eastern Syria, home to most of Syria's main oilfields.

Experts say the extremists were earning as much as US$3 million (S$3.8 million) daily from oil before the coalition began launching strikes on Syria, building on the air war under way against IS in Iraq since Aug 8.

Seven of the US strikes were east of Diban; they "successfully struck an ISIL (IS) crude oil collection point consisting of crude oil collection equipment and a modular oil refinery," the statement said.

Another of the five others against oil targets was south-east of Deir Ezzor and damaged a modular refinery.

Three other strikes in Syria hit two IS fighting positions near Kobane and a military camp in the Raqa province.

In Iraq, five air strikes south and west of Bayji "struck two ISIL units, destroyed one ISIL armed vehicle, an ISIL heavy weapon, an ISIL machine gun position, an ISIL building and three ISIL guard shacks and damaged one ISIL guard shack."

Another five west of the strategic Mosul Dam destroyed vehicles and damaged a building occupied by militants.