U.S. President Barack Obama.
U.S. President Barack Obama may have averted a fiscal crisis for now, but the aftereffects will take time to wear off. Photo by AP
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U.S. President Barack Obama warned lawmakers on Tuesday that Iran would make progress in its ability to build a nuclear weapon if there is no diplomatic deal to halt or roll back its nuclear program, and urged Congress to hold off on tightening sanctions against Tehran while talks continue.

"The president underscored that in the absence of a first step, Iran will continue to make progress on its nuclear program by increasing its enrichment capacity, continuing to grow its stockpile of enriched uranium, installing advanced centrifuges, and making progress on the plutonium track," White House spokesman Jay Carney told reporters at a briefing.

Senators urge Kerry to play hard

Earlier on Tuesday, a bi-partisan group of U.S. senators authored a letter to U.S. Secretary of State John Kerry, in which they expressed their support for the negotiations - but cautioned against accepting a deal ''that would roll back economic sanctions without also rolling back progress towards nuclear weapons capability."

Commending the efforts of Kerry and his team, the letter urged negotiators "to fight for an interim agreement that demands as much or more of Iran as it does of the United States and our allies."

The letter reminds Kerry to ''be mindful" of his negotiating partners' deeds, past and present. "Iran has been the largest state sponsor of terrorism for over thirty years; its leaders routinely call for the destruction of Israel; and it arms and finances terrorist groups around the globe."

Concluding in a hopeful note, the senators expressed their hope that in "the next few weeks we and our partners will redouble our efforts to gain greater proportionality and to finalize an agreement that demonstrates that Iran is moving away from the nuclear weapons path."

The letter was signed by senators Charles E. Schumer, Lindsey Graham, Robert Menendez, John McCain, Bob Casey and Susan Collins.