Tehran is in the thick of international negotiations over its nuclear program, with a fresh round of talks beginning on today in Vienna aimed at starting a draft of an historic final deal.
Under a temporary deal which took effect January 20, Iran froze certain nuclear activities for six months in exchange for minor relief from sanctions hurting its economy.
A full-bore resumption of enrichment -- a gross violation of the temporary accord -- could see Tehran move swiftly toward nuclear breakout.
Senate Foreign Relations Committee chairman Robert Menendez cited reports that said UN Security Council permanent members Britain, China, France, Russia and the United States, along with Germany, should focus on extending the time it would take for Iran to produce nuclear weapons to between six and 12 months.
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Kerry said the ultimate goal was assurance that Iran never build an atomic bomb.
"So six months to 12 months is -- I'm not saying that's what we'd settle for -- but even that is significantly more" than the estimated two months to breakout, he said.
Tehran last month insisted that the reactor, a concern to the West because Tehran could extract weapons-grade plutonium from its spent fuel if it also builds a reprocessing facility, will not be torn down.
Menendez said Arak remained part of a "worrisome" effort by Tehran to advance its nuclear program in the midst of the negotiations.
"Their research and development capacity (is) still moving forward as we speak, which only allows them to create more sophisticated centrifuges, which closes the window for them even more quickly" towards breakout capacity, Menendez told Kerry.