WASHINGTON — A Luxembourg bank paid a $152 million fine for masking the Bank of Iran’s $2.8 billion U.S. securities account.

“Clearstream provided the Government of Iran with substantial and unauthorized access to the U.S. financial system,” Adam Szubin, the director of the Department of Treasury’s Office of Foreign Assets Control, said in a Jan. 23 statement, referring to Clearstream Banking, S.A.

“Today’s action should serve as a clear alert to firms operating in the securities industry that they need to be vigilant with respect to dealings with sanctioned parties, and that omnibus and custody accounts require scrutiny to ensure compliance with relevant sanctions laws,” he said.

According to the statement, Clearstream in 2008 said after meeting with Treasury officials that it would end its relationship with the Bank of Iran, which is subject to U.S. sanctions in part because of its involvement in Iran’s suspected nuclear weapons program. Instead, Clearstream disguised the account, hiding the holdings in the account of a European bank unnamed in the statement.

David Cohen, the Treasury undersecretary who supervises sanctions monitoring and enforcement, told The New York Times that the fine is part of a message that Iran is “not open for business” despite limited sanctions relief triggered by Iran’s agreement in November to negotiate its nuclear capacity.