U.S. Secretary of State John Kerry leaves the bilateral meeting with Iranian Foreign Minister Mohammad Javad Zarif (not pictured) for a new round of Nuclear Iran Talks, in Montreux, Switzerland. (Jean-Christophe Bott/EPA)

Secretary of State John F. Kerry directly rebutted critics of nuclear talks with Iran, saying Wednesday that demands for more sanctions will not slow Iran’s nuclear program as much as the agreement being pursued.

Kerry’s comments echoed statements from President Obama and marked his first ­public response to a Tuesday speech to Congress by Israeli Prime Minister Benjamin Netanyahu, who denounced the U.S.-led efforts to strike an agreement to rein in Iran’s nuclear program.

Kerry, wrapping up three days of talks with Iran’s foreign ­minister, said he would not allow politics or “external factors” to distract him from the negotiations as they moved toward a possible make-or-break juncture at the end of the month.

“The first step is to prevent Iran from acquiring a nuclear weapon,” Kerry told reporters in the Swiss resort town of ­Montreux. “And we know that absent a deal, Iran will have the ability to move ahead with its nuclear program. That we know for sure, because that’s exactly what’s happened to date.”

The ultimate aim is to ensure that Iran’s nuclear proficiency is directed only toward peaceful ends. The West and its allies worry that Iran’s ability to enrich uranium could one day be expanded to produce material for a nuclear weapon. Iran says it seeks nuclear fuel only for energy-producing reactors and medical uses.

Graphic: Iran's potential nuclear capability

[Factcheck: Netanyahu takes Kerry out of context]

In over a decade of talks and sanctions, Kerry said, the only measure that effectively slowed Iran’s nuclear expansion has been the interim agreement in place since November 2013 while the latest negotiations have been underway.

“No one has presented a more viable, lasting alternative for how you actually prevent Iran from getting a nuclear weapon,” he said, noting that sanctions hurt Iran enough to get it to the negotiating table but not to stop its nuclear expansion. “So, folks, simply demanding Iran capitulate is not a plan.”

“We continue to be focused on reaching a good deal, the right deal,” he added.

He spoke shortly after finishing his last round of talks with Iranian Foreign Minister Mohammad Javad Zarif. Kerry arrived in Saudi Arabia a few hours later to meet with the newly crowned King Salman and foreign ministers from other Persian Gulf Arab countries, which view Iran as a major regional rival.

On Saturday, Kerry plans to meet with the foreign ministers of France, Britain and Germany in Paris.

Talks with Iran are scheduled to resume March 15.

Jason Rezaian’s journey has taken him from a childhood in San Francisco to his father’s native Iran. At 37, he became the Washington Post correspondent in Tehran. In July 2014, he was thrown into Iran’s Evin Prison, where he remains, without access to a lawyer. This is his story. (Jorge Ribas/The Washington Post)

Negotiators for the United States, the European Union and Iran are racing to reach at least a general understanding by the end of March. Then it will be up to Obama to decide whether enough progress has been made for the talks to continue with the goal of ironing out technical details by June 30.

Although U.S. and Iranian officials said some progress was made in the latest dialogue, there was no suggestion of a breakthrough.

“We are still working through some difficult issues,” said a senior State Department official, without giving details, who spoke to reporters on the condition of anonymity under agency rules.

Asked whether the two sides had made progress, Zarif told reporters, “We have, but a lot of work remains,” according to the Reuters news agency.

In Washington, Netanyahu sought to rally opposition to a possible deal in his address to a joint meeting of Congress on Tuesday. None of the negotiators in Montreux watched his address live, though some have seen clips of his remarks, the official said.

[Read: Netanyahu warns that deal paves Iran’s path to a bomb]

The negotiators are under intense pressure to come up with a deal that gains wide acceptance, not only in Israel but also in other countries in the region worried about Iran’s expanding influence.

“Now, we still don’t know whether we will get there, and it is certainly possible that we won’t,” Kerry said. “It may be that Iran simply can’t say yes to the type of deal that the international community requires. But we do know that we owe it to the American people, in my case, [and the] people in the world to try to find out.”

The U.S. delegation at the Wednesday talks included Energy Secretary Ernest Moniz, and the Iranian team included Ali Akbar Salehi, the atomic energy chief.

But more action seemed to be happening behind the scenes.

Kerry spokeswoman Jen Psaki sent reporters a copy of an article published by Factcheck.org that said Netanyahu had misrepresented Kerry in one of the central points of his speech.

The vetting Web site said Netanyahu had taken Kerry out of context when he claimed that Kerry “confirmed last week that Iran could legitimately possess” 190,000 centrifuges, used to enrich uranium, as part of the agreement the United States is trying to negotiate with Iran. This, the Israeli leader said, could put the Islamic republic “weeks away” from the ability to build up an “arsenal of nuclear weapons.”

In fact, Kerry spoke more generally in testimony he gave to the House Committee on Foreign Affairs on Feb. 25, saying that a “civilian power plant that’s producing power legitimately, and not a threat to proliferation, you could have as many as 190,000 or more centrifuges.” Deputy spokeswoman Marie Harf said at the time that he was not speaking specifically about Iran.

Psaki’s e-mail made no additional comments, beyond the subject line, “Please share.”

Daniela Deane in London and Brian Murphy in Washington contributed to this report.