"What we most care about is how Iran responds to this in terms of its preparedness to engage in negotiations," said Steinberg. "Our goal, as I said, is not sanctions for sanctions sake, but to make clear to Iran that there are costs for the path they are pursuing. But I think we have pretty good indications that - not just in the energy sector but across-the-board - that whether it's in the financial sector, whether it's in shipping or transportation - that these measures are increasingly having a significant impact on Iran."

Steinberg said the administration is conducting investigations of other foreign firms that have not yet pledged to stop doing business in Iran.

He also said the United States is urging other countries to bar their oil companies from stepping in and replacing the European firms that have promised to sever Iranian ties.

On Wednesday, the Obama administration announced targeted sanctions against eight Iranian political figures including the country's Revolutionary Guards Corps commander, for political repression in the aftermath of disputed elections last year.

The travel and financial penalties, recently authorized by an act of Congress, are the first imposed by the United States for human rights abuses.

Steinberg said he discussed Iran's "destabilizing activities" in the Middle East, including threats against Israel, in a meeting earlier Thursday with Syrian Deputy Foreign Minister Fayssal Mekdad.