WASHINGTON, Oct. 23 -- The Islamic State (IS) group is amassing wealth at an "unprecedented" pace, generating tens of millions of dollars per month, a U.S. official said on Thursday.

The militant group "poses a different terrorist financing challenge," as its revenue sources have a different composition from many other terrorist organizations, said David Cohen, the Treasury undersecretary for terrorism and financial intelligence.

The group's primary funding tactics include the sale of stolen oil, the ransoming of kidnap victims, theft and extortion, as well as donations from supporters, Cohen said in a speech at the Carnegie Endowment for International Peace.

"With the important exception of some state-sponsored terrorist organizations, ISIL is probably the best-funded terrorist organization we have confronted," Cohen said, using an alternate acronym of the group.

The U.S. official said it is difficult to get precise revenue estimates on the value to IS of the transactions in light of the murky nature of the market, but the U.S. estimates that beginning in mid-June, the group has earned about 1 million U.S. dollars a day from black market oil sales.

Ransom payments are irregular for IS, but each one can be a significant boon, said Cohen, adding that the group has taken at least 20 million dollars in ransoms this year.

Unlike the al-Qaida terrorist network, IS obtains the vast majority of its revenues through local criminal and terrorist activities, Cohen said, adding that U.S. Treasury Department's tools are not particularly well-suited to combating extortion and local crime.

"We are focused on restricting ISIL's access to the international financial system in order to impair its ability to collect funds from abroad, and to move, store, and use the funds it acquires locally," he said.

The official said that the efforts to combat IS's financing will take time, as with the rest of the campaign against the group.

"We have no silver bullet, no secret weapon to empty ISIL's coffers overnight," he said. "This will be a sustained fight, and we are in the early stages."