WASHINGTON - US Secretary of State John Kerry mounted a furious counterattack against critics of the Iran nuclear deal on Thursday, telling skeptical lawmakers that rejection of the accord would give Tehran "a great big green light" to swiftly accelerate its atomic program.Testifying before Congress for the first time since Iran and world powers reached the deal last week, Kerry fought back against accusations by a senior Republican that America's top diplomat was "fleeced" by Iranian negotiators in the final round of the Vienna talks.He insisted that those who oppose the deal, which curbs Iran's nuclear program in return for sanctions relief, are pushing an unrealistic alternative that he dismissed as a "sort of unicorn arrangement involving Iran's complete capitulation.""The fact is that Iran now has extensive experience with nuclear fuel cycle technology," Kerry told the Senate Foreign Relations Committee. "We can't bomb that knowledge away. Nor can we sanction that knowledge away."Kerry said that if Congress turns thumbs down on the deal, "the result will be the United States of America walking away from every one of the restrictions we have achieved.""We will have squandered the best chance we have to solve this problem through peaceful means," he said as Congress began a 60-day review of the deal to decide whether to support or reject it.