The United States has designated four companies and one person as violators of U.S. oil sanctions on Iran.

Another 12 companies and people have been identified as front companies aiding Iran's nuclear program.

The State Department and U.S. Treasury announced the moves today, nearly one month after reaching a deal with Iran and world powers in Geneva Nov. 24, which will include rolling back sanctions in exchange for limits on nuclear-enrichment progress and reactor construction in Iran.

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"Today's actions should be a stark reminder to businesses, banks and brokers everywhere that we will continue relentlessly to enforce our sanctions, even as we explore the possibility of a long-term, comprehensive resolution of our concerns with Iran's nuclear program," Under Secretary for Terrorism and Financial Intelligence David S. Cohen said in announcing the designations.

Under these designations, the companies won't be able to do business with Americans, any assets under U.S. jurisdiction will be frozen and foreign entities will risk penalties for doing business with them. It's unclear whether they can challenge the designations.

They are the second batch of designations since this summer when Iran elected its new president, Hassan Rouhani, a self-proclaimed moderate who has paved the way for negotiations and, for the time being, improved relations with the West.

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Treasury had slowed its enforcement of sanctions on Iran since Rouhani's election, some members of Congress have insinuated. During that time, the United States was holding secret talks with Iranian officials, the State Department has acknowledged, and The Daily Beast's Eli Lake and Josh Rogin reported in November that enforcements had slowed to a crawl since the new Iranian president took office.

U.S. officials have said that sanctions enforcements happen as violations occur, and that volume isn't always a reliable measure of vigilance.

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"We have continued to pursue sanctions enforcements and sanctions designation activity post election of Rouhani just as we were prior to the election of Rouhani," a senior administration official told reporters on a conference call today.

Treasury last announced new Iran-sanctions violators Sept. 6, when it accused six individuals and four companies.

Included on today's list of alleged oil-sanctions violators were a petrochemical marketing and distribution firm, Mid Oil Asia; two tanker companies, Singa Tankers and Siqiriya Maritime; a Ukrainian energy company, Ferland Company Limited; and its general manager, Vitaly Sokolenko.

The Obama administration has sought to convince Congress and the public that its recent deal with Iran will not mean any lessening of sanctions that will remain on the books, and senior administration officials stressed that point in a conference call with reporters explaining the deal.

Republicans and Democrats in Congress have called for new sanctions, while Iranian Foreign Minister Javad Zarif has said that any new sanctions bill, even one timed to the end of the interim nuclear agreement - would mean the deal is off. Senate Banking Committee Chairman Tim Johnson, D-S.D., reaffirmed at a hearing today that his committee will hold off on new sanctions until the six months of negotiations have concluded.

Johnson has, however, drafted a bill with new sanctions against Iran that his committee is prepared to finalize if the deal with Iran falters.

"I support strong sanctions, and authored many of the U.S. sanctions currently in place," Johnson said at today's hearing. "I have negotiated a new bipartisan sanctions bill with my Ranking Member that could be finalized and moved quickly if Iran fails to comply with the terms of the first step agreement in Geneva, or if negotiations collapse.

"Sanctions have been an effective tool of coercive diplomacy, crippling Iran's economy, sharply curtailing its oil revenues, and helping to persuade the Iranian people to vote for new leadership."

Several lawmakers, including Sens. Bob Menendez, D-N.J., and Mark Kirk, R-Ill., are working on a separate bill that would impose new sanctions on Iran if talks fail.