(Updates throughout with quotes from Kerry and McCain, details and background)
By Patricia Zengerle
WASHINGTON, April 8 (Reuters) - Iran can produce fissile material for an atomic weapon in two months, U.S. Secretary of State John Kerry on Tuesday told a Senate hearing in which he faced tough questions from lawmakers about negotiations with Iran over its nuclear program.
"I think it's public knowledge today that we're operating with a time period for a so-called breakout of about two months. That's been in the public domain," Kerry testified at a Senate Foreign Relations Committee hearing.
"So six months to 12 months is - I'm not saying that's what we'd settle for, but even that is significantly more," Kerry said in response to a question about whether a "breakout" window of up to a year was the negotiators' goal.
The United States and five other world powers began a new round of talks with Iran in Vienna on Tuesday to try to reach a broad settlement over its disputed nuclear program, which Tehran denies is aimed at building a bomb.
Iran and the six powers agreed an interim deal in November under which it curbed some uranium enrichment activities in return for limited relief from sanctions imposed for its nuclear program.
Kerry said such a "breakout" window did not mean Iran would have a warhead or other delivery system. "It's just having one bomb's worth, conceivably, of material, but without any necessary capacity to put it in anything, to deliver it, to have any mechanism to do so," he said.
Several committee members, both Democrats and Republicans, take a harder line on the Islamic Republic than many administration officials, and have voiced deep skepticism about prospects for the talks under way in Vienna.
Many are deeply unhappy that President Barack Obama's administration may sign an agreement with Tehran that would allow it any uranium enrichment.
"On the major issues, this administration is failing very badly," said Republican Senator John McCain of Arizona.
Several committee members, including its chairman, Democrat Robert Menendez, backed the imposition of new sanctions, even though administration negotiators have said any new economic restrictions could endanger the talks.
Kerry said Washington and its allies would be prepared to act if Iran were to produce fissile material, and not just with sanctions.
"If they're overtly breaking out and breaking an agreement and starting to enrich and pursue it, they've made a huge consequential decisions. And the greater likelihood is we are going to respond immediately," Kerry said. (Additional reporting by Arshad Mohammed; Editing by Doina Chiacu and Mohammad Zargham)