Update (11:30 p.m. EST): President Barack Obama wrote that the new sanctions, which extend to foreign branches of U.S. banks, came as Iranian banks tried to conceal transactions with parties already sanctioned by the United States. In a letter to Congress Monday he wrote:

I have determined that additional sanctions are warranted, particularly in light of the deceptive practices of the Central Bank of Iran and other Iranian banks to conceal transactions of sanctioned parties, the deficiencies in Iran's anti-money laundering regime and the weaknesses in its implementation, and the continuing and unacceptable risk posed to the international financial system by Iran's activities.

Original: A new executive order the president just signed puts a hold on all Iranian assets in the United States, including those of the country's central bank, that blocks them from export, transfer, or any other movement, as part of a new round of sanctions by the government.