WASHINGTON - Secretary of State John Kerry defended an international deal to restrict Iran's nuclear program to skeptical lawmakers Tuesday, arguing that more sanctions and less dialogue would backfire and make Tehran only more likely to pursue weapons capabilities.

Kerry, testifying before the House Foreign Affairs Committee, said that voting for more sanctions now was "gratuitous" because Iran already stood to lose more than $30 billion in the next six months as a result of sanctions, compared with the $7 billion in sanctions relief it would get under the deal.

He added that his Iranian counterpart, Foreign Minister Javad Zarif, had boasted to him that Iran had managed to operate 19,000 centrifuges despite existing sanctions and international rebukes.

"What do you get by not talking?," Kerry warned. "You get closer to a bomb."

Kerry's tone in the nearly two-hour hearing was alternately defiant and pleading as he lobbied Congress to scrap plans for additional penalties on Iran during the first six-month phase of the agreement, which was brokered last month in Geneva by the United States and fellow U.N. Security Council permanent members China, France, Russia, and the United Kingdom, plus Germany.

Kerry said that imposing deeper sanctions now would send a signal to those partners that the United States wouldn't uphold its end of the deal. And, he said, such action would embolden Iranian hardliners who oppose relatively moderate President Hasan Rouhani's overtures to the West and would only make the regime more intent on flouting international inspectors and going after a bomb.

On the other hand, Kerry suggested, if all went well, there was an "outside chance" that a final, comprehensive nuclear deal could be reached even before the six-month mark.

"Let me be very clear: This is a very delicate diplomatic moment," he said, "and we have a chance to address peacefully one of the most pressing national security concerns that the world faces today."

It's uncertain whether Kerry was able to sway the legislators. A bipartisan bloc has drafted sanctions legislation to attach to the annual defense authorization bill, which could be put to a vote before Congress breaks for the holidays.

At the hearing, both Republicans and Democrats voiced a litany of similar concerns: that the deal jeopardizes Iran's archenemy, Israel; that there's no basis for trusting a pariah state that's deemed a sponsor of terrorism; and that the deal allows Iran to keep enriching uranium.