London, UK (Reuters) - The Islamic

London, UK (Reuters) - The Islamic State (IS) militant group has been forced to cut its fighters' pay by up to 50 percent because U.S.-led air strikes have had a substantial impact on the money it makes from oil, a senior U.S official said on Monday.

Daniel Glaser, assistant secretary for terrorist financing at the U.S. Treasury Department, said the strikes had hit the group's ability to extract, refine and transport oil from territory it controlled in Iraq and Syria.

"When you look at difficulties that we know that they are having with respect to the transport, with respect to the extraction, I think it's fair to say they are no longer able to make money the way they used to be able to," Glaser told a London conference.