His scheduled testimony Tuesday before the House Foreign Affairs Committee comes as lawmakers threaten to undermine last month's interim nuclear accord in Geneva. There, the U.S. and world powers promised Iran $7 billion in sanctions relief in exchange for nuclear concessions.
The deal prohibits the Obama administration from introducing new sanctions for six months. Kerry and other U.S. officials have warned of dire consequences if Washington breaks its word. And Iran's foreign minister, Mohammad Javad Zarif, has said any new package of commercial restrictions would kill the deal.
"If Congress adopts sanctions, it shows lack of seriousness and lack of a desire to achieve a resolution on the part of the United States," Zarif told Time magazine. "My parliament can also adopt various legislation that can go into effect if negotiations fail. But if we start doing that, I don't think that we will be getting anywhere."
Kerry also is concerned new legislative action would be a sign of bad faith to America's negotiating partners, which were Britain, France, Germany, China, Russia. The U.S. is banking on them to enforce existing oil and financial restrictions on Tehran and to press Iran into a final nuclear deal next year.
Appearing at a weekend think-tank forum, President Barack Obama said he believed the chances for such a comprehensive nuclear agreement are 50-50 or worse.
Still, he defended diplomacy as the best way to prevent Tehran from acquiring atomic weapons. He also rejected criticism from Israel's government and many in Congress that his administration bargained away too much without securing a complete halt to Iran's nuclear program — as demanded by the international community for several years.
Obama said the interim agreement forces the Islamic republic to eliminate higher-enriched uranium stockpiles, stop adding new centrifuges and cease work at a heavy water reactor that potentially could produce plutonium. It also provides time to see if the crisis can be averted through negotiation, he said.
Iran insists its program is solely for peaceful nuclear energy and medical research.
In Washington, Republican and Democratic lawmakers are leery.
They credit crippling petroleum, banking and trade sanctions levied on Iran in recent years with bringing its more moderate president, Hassan Rouhani, to power and his representatives to the negotiating table. They argue more pressure, not less, could break Iran's will and secure better terms in a final agreement.
"We have bargained away our fundamental position," said Rep. Ed Royce, R-Calif., the House Foreign Affairs Committee chairman. "Iran should not be enriching and reprocessing," he said, criticizing what he termed the administration's "false confidence that we can effectively check Iran's misuse of these key nuclear bomb-making technologies."
Sens. Bob Menendez, D-N.J., and Mark Kirk, R-Ill., are close to completing a new bill that would require the administration to certify every 30 days Iran's adherence to the interim pact, according to legislative aides.
Without that certification, sanctions worth more than $1 billion a month would be re-imposed and new restrictions on Iran's engineering, mining and construction industries would be added. The legislation also calls for a global boycott of Iranian oil by 2015. Foreign companies and banks violating the sanctions would be barred from doing business in the United States.
House Majority Leader Eric Cantor, R-Va., is drafting similar legislation, aides say.
Despite Zarif's declaration and Kerry's warnings, it's unclear if suspended sanctions from Congress would necessarily break the Geneva agreement. Perhaps conscious of the looming struggle with lawmakers, the Obama administration pledged somewhat ambiguously to refrain from imposing new nuclear-related sanctions "consistent with the respective roles of the president and the Congress."
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