The Obama administration intensified efforts on Thursday to counter what officials called a misimpression that the six-month nuclear agreement with Iran had opened the door to new economic opportunities with the country, emphasizing that nearly all sanctions remained in force and warning businesses not to engage in any deals still pending after the accord’s July 20 expiration.

As if to punctuate the administration’s assertion that little had changed, the Treasury Department announced what it described as a landmark $152 million settlement with Clearstream Banking, a Luxembourg-based banking subsidiary of Germany’s Deutsche Börse securities exchange, for having allowed Iran to bypass sanctions through the use of the company’s access to the American banking system.

“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,” Adam J. Szubin, director of the Treasury’s Office of Foreign Assets Control, which helps to police compliance with American sanctions, said in a statement announcing the settlement.

The administration has been facing increased criticism from supporters of strong sanctions against Iran who contend that the six-month deal — which went into force on Monday and was devised to allow time to negotiate a permanent accord — had given the Iranians far more in economic benefits than what its provisions had specified or intended.