WASHINGTON (Reuters) - A recent letter attributed by the United States to top al Qaeda official Ayman al-Zawahri is a fresh sign the militant network is facing financial troubles, a senior U.S. Treasury official said on Wednesday. "There's ... a passage in there where Zawahri says to Zarqawi: I'm out of money. Send me money. I don't have any money," said Stuart Levey, undersecretary for terrorism and financial intelligence, referring to Abu Musab al-Zarqawi, the leader of al Qaeda in Iraq. "I'm a glass-half-full kind of guy, so I look at that as a good thing. Zawahri's out of money. Glass-half-empty people would say, well, he thinks Zarqawi has got so much money he's got it to spare," he said in a speech at the Heritage Foundation think tank. U.S. intelligence officials released the purported letter from Zawahri to Zarqawi last month, but al Qaeda's wing in Iraq rejected it as a fabrication. The letter has Zawahri telling Zarqawi: "We need a payment while new (financial supply) lines are being opened. So, if you are capable of sending a payment of approximately one hundred thousand (dollars), we'll be very grateful to you." U.S security officials have repeatedly said they are confident of the letter's authenticity, but some U.S. terrorism experts say its style and some of the content -- including the request for money -- made them concerned it might be a fake. Levey said while he could not reveal specifics, there was other anecdotal evidence that tough measures to curb terrorism funding had put the screws on al Qaeda as well as Palestinian militant group Hamas over the past few months. The undersecretary said in May that al Qaeda and Hamas were having difficulties collecting, transferring and storing funds. Hamas has told Reuters it has been hit hard by a drop in revenue following a Saudi crackdown on the abuse of charities and other avenues of financing. "Anecdotally what we're seeing is that we are having a real impact on al Qaeda and Hamas, both in terms of putting pressure on them financially but also in terms of creating deterrents both for donors to give money to them and how they're able to move money," Levey said in the Wednesday speech. The United States has taken a series of steps since the September 11, 2001, attacks -- including freezing funds, requiring banks to report more suspicious transactions and making informal cash brokers register with authorities -- to make it harder for militants to abuse the U.S. financial system.