It's unclear how well non-military means in Iraq and Syria will diminish the revenue base for the Islamic State, a scholar from RAND Corp. testified.

Members of the House Committee on Financial Services heard testimony on oil revenue for the Sunni-led terrorism group in control over parts of Iraq and Syria.

At the height of its campaign, IS controlled as many as seven oil fields and was said to be generating as much as $2 million per day in oil revenue. David S. Cohen, undersecretary for terrorism and financial intelligence at the Department of the Treasury, testified the group is "probably now earning several million dollars per week" from smuggled oil.

Patrick Johnson, a counter-terrorism specialist at RAND Corp., testified it was difficult to understand the full complexity of the IS smuggling ring in the region.

"Although some information exists on the group's oil sales and the smuggling schemes that are used to move oil to market in Turkey, Kurdistan, and elsewhere, getting the high-fidelity comprehensive intelligence on how IS sells oil and smuggles it will be difficult," he remarked.

Cohen in his testimony said financial sanctions on specific targets and coordination with regional allies was taking a toll on revenue streams for IS. With no combat forces on the ground, airstrikes are further diminishing daily revenue for the group, he said.

For Johnson, non-military options may be limited because of "opacity of the informal economic processes" the group uses to profit from oil.

"After all, IS runs that part of the world and claims to have its own state," he said. "Thus, disrupting the group's oil revenues through sanctions may be possible, but in a less precise fashion than we would like."