The man responsible for aggressive economic sanctions against Iran, al-Qaida, North Korea and other terrorist groups is leaving his post at the Treasury Department.

Stuart Levey's resignation letter is expected to be on President Obama's desk today, and he'll leave his post as Under Secretary for Terrorism and Financial Intelligence in a month, the Wall Street Journal reports. Levey's longtime assistant David Cohen is expected to be nominated as his replacement.

Levey has held the position since 2004. Under his leadership, the terror-financing section became "a major cog in the U.S. national-security apparatus," the Journal's Jay Solomon wrote, with hundreds of people tracking international financing and export license applications. He has been tireless in enlisting international cooperation for shutting off banking access to terrorist financiers and rogue states like Iran and North Korea.

He has not indicated what prompted his departure or what he intends to do next. He pledged to stay at Treasury for six months when the Obama administration came into office, said Treasury Secretary Tim Geithner. Levey's departure "will have no effect on policy, or on our ability to execute the President's policy."

Cohen inherits significant challenges, especially after the latest round of international talks over Iran's nuclear program fell apart Saturday. Iranian officials demanded the existing sanctions be suspended as a pre-condition, and the talks went nowhere.