U.S. airstrikes on Islamic State targets in parts of northeastern Syria include strategic control structures and oil refineries, the State Department said.

Militants with the group calling itself the Islamic State may be on the verge of claiming control over the strategic Turkish-Syrian border town of Kobani. U.S. airstrikes have targeted Islamic State militants in the region, though U.S. State Department spokeswoman Jen Psaki said the "primary objective" was to prevent the group from gaining a safe haven in the area.

"Our focus strategically is on command and control structures, oil refineries, and that's where we're taking our military action," she said during her regular press briefing Tuesday.

The Sunni-led terrorist group is said to be generating about $2 million per day on pilfered oil. Iraqi leaders and peace advocacy groups have said action must be taken to contain oil's role in IS financing.

Last week, the Energy Intelligence Group said U.S.-led military efforts have knocked out about half of the refinery capacity captured by the group.

The report said the biggest loss for the militant network is the destruction of its oil processing capabilities in Syria.