At a news conference in Geneva, Secretary of State John Kerry said of a nuclear deal with Iran: “This can be done. I’m not going to tell you it will be. But I can absolutely tell you it can be.” (Jason Reed/AP)

Secretary of State John F. Kerry heads home Monday to defend a proposed nuclear deal with Iran in testimony before doubting lawmakers, as the Obama administration moves to head off growing criticism from Israel.

Kerry has already begun making the case that an Iranian agreement to temporarily freeze elements of its nuclear programs in exchange for a partial easing of Western sanctions would be a viable step toward negotiating a permanent end to any Iranian nuclear weapons ambitions.

If negotiators in the next several weeks can reach an agreement on the draft proposal — a result that eluded top world diplomats in intensive sessions with the Iranians that ended here Sunday — talks can move toward making “absolutely certain . . . that Iran never has a nuclear weapon,” Kerry said in an interview on NBC’s “Meet the Press.”

Far from being naive about Iran’s capabilities and intentions, as some opponents have suggested, Obama administration negotiators are “some of the most serious and capable, expert people in our government, who have spent a lifetime dealing with both Iran” and nuclear proliferation issues, Kerry said.

“We are not blind, and I don’t think we’re stupid,” he added. “I think we have a pretty strong sense of how to measure whether or not we are acting in the interests of our country and of the globe.”

Congress is threatening to push through additional financial sanctions against Iran, a move that could complicate the talks.

For those convinced that Iran still has every intention of building a nuclear weapon, the relaxed demeanor of Iranian Foreign Minister Mohammad Javad Zarif at a brief news conference here early Sunday seemed to prove their point. Despite the failure to conclude the deal, Zarif said, he found “political determination, willingness and good faith” among his diplomatic counterparts.

Iran would not be so publicly cheerful, Israeli officials and doubting U.S. lawmakers argued, if it was not on the verge of winning a deal that was bad for their side.

But an additional reason for Zarif’s smiling banter with journalists may have been that those across the table from the Iranian team reportedly spent at least as much time debating one another as they did him.

“Obviously, the six countries may have differences of views,” Zarif said benignly of his counterparts from the United States, France, Britain, Germany, Russia and China. “But we are working together, and hopefully we will be able to reach an agreement when we meet again” 10 days from now.

‘Dangerous . . . for us’

Israeli Prime Minister Benjamin Netanyahu, who has kept up a steady drumbeat of public criticism, said Sunday that he has begun calling world leaders to convince them that the proposed agreement would be “dangerous not just for us; it is also dangerous for them.”

“I emphasized that the proffered deal does not include the dismantling of even one centrifuge. I asked all the leaders, why the haste?” Netanyahu said at a commemoration for Israel’s founding father and first prime minister, David Ben Gurion. “I proposed that they wait, that they consider the matter seriously.”

In a letter sent Saturday night to major Jewish organizations around the world, including in the United States, Israeli Diaspora Affairs Minister Naftali Bennett urged them to pressure their governments not to sign the deal.

U.S. officials think there is no chance that Iran, absent a phased, negotiated deal, will ever give in to sanctions pressure. But Israeli concerns are shared by many in Congress, and the Obama administration is treating them gingerly.

On Sunday, the administration dispatched Wendy Sherman, undersecretary of state for political affairs, who has headed the U.S. negotiating team, to Israel to explain more details of the proposed agreement and to repeat President Obama’s pledge that he will never allow Iran to obtain a nuclear weapon.

‘This can be done’

Kerry broke off a week-long Middle East trip to travel here, along with his fellow foreign ministers, after negotiating teams said they had reached a point where the participation of top diplomats could push a draft deal to completion.

From Geneva, Kerry headed to Abu Dhabi for a brief visit. He plans to arrive home late Monday in advance of congressional testimony Tuesday.

At his own news conference early Sunday, Kerry said adjournment of the high-level talks without success is simply a pause, not a failure. The draft document is complicated, he said, and the negotiating pace has been intense. Participating governments need to study it and explain it to their officials and publics before returning here later this month to resume their talks and hopefully reach an agreement, he said.

“There is no question in my mind that we are closer now to a deal” than diplomats were last week, Kerry said. “This can be done,” he added. “I’m not going to tell you it will be. But I can absolutely tell you it can be.”

Although Zarif has his own doubters among hard-liners at home, the Iranians seemed largely on board with the broad outlines of the proposal.

But in addition to Israel and Congress, Kerry has some work to do among his negotiating partners. Officials on their side of the table suggested that disagreements, raised most prominently by France, came down to a handful of what some thought were imprecise and unclear words on the specifics of the deal. The draft had left the scope and size of some elements for the foreign ministers to finalize.

‘A fool’s game’

In general, the interim proposal leaves the basic architecture of Iran’s programs and Western sanctions in place and capable of being fully resumed, while freezing the former and easing the latter. During a six-month period, the two sides would negotiate a final agreement to permanently eliminate Tehran’s ability to develop a nuclear weapon and fully restore Iran to the community of nations.

Discussions centered on the extent to which activities would be suspended at Iran’s Arak heavy-water reactor, which is under construction and is designed eventually to produce weapons-grade plutonium. Would activity at the site be frozen before delivery of fuel and other components, for example, or just before start-up, and how would it all be verified?

Concerning Iran’s other “breakout” path to weapons production — high-potency enrichment of uranium — there were questions about the extent to which Iranian centrifuges could continue to operate during the interim period and what to do with its stockpile of uranium already enriched to 20 percent. Iran maintains that it is uninterested in nuclear weapons and that its programs are all directed toward ensuring an energy supply.

On the sanctions side of the deal, the yet-to-be-agreed-upon specifics include granting Tehran access to a certain amount of frozen cash and easing pressure on countries that continue to buy Iranian oil exports.

Some ministers also expressed concern over the precise extent and wording of sanctions-relief language, noting that many U.S. economic sanctions require legislative action and that they would probably be called upon to make the initial moves.

Reports that France had shared Israel’s assessment drew condemnation in the Iranian media and gratitude from Israel, where French President François Hollande is to address the Knesset next week.

Beyond whatever concerns France may have expressed in the closed-door meetings, other negotiators appeared irritated at comments that French Foreign Minister Laurent Fabius made to reporters who milled around the lobby of the Geneva InterContinental Hotel, where the talks were being held.

On Friday, Fabius told French radio that the proposed deal was a “fool’s game” that was too advantageous to Iran. After the last round of discussions early Sunday, as European Union foreign policy chief Catherine Ashton, who hosted the negotiations, and Zarif were composing a carefully worded announcement saying progress was made and another high-level meeting would soon be convened, Fabius left the room and told waiting reporters that there was no deal.

William Booth in Jerusalem and Joby Warrick in Washington contributed to this report.