The nuclear programme of Iran

UNITED NATIONS—Israeli Prime Minister Benjamin Netanyahu on Thursday delivered a fiery address here condemning the Iranian nuclear deal, largely unbowed in his opposition despite losing steep political ground to President Barack Obama over the issue this year.

In his speech to the General Assembly, Mr. Netanyahu thundered that Iranian threats to destroy Israel have been met in the world body by “utter silence, deafening silence.”

He then stopped speaking for 45 seconds, panning the hall with a furrowed glare.

“Perhaps you can now understand why Israel is not joining you in celebrating this deal,” he said.

The nuclear deal, reached in July between Iran and six world powers including the U.S., passed a crucial milestone when the U.S. Congress failed to adopt a resolution of disapproval that essentially could have blocked the deal from moving forward.

Mr. Netanyahu had thrown his support behind congressional opponents of the deal, delivering a controversial speech to Congress in March and meeting repeatedly with U.S. lawmakers.

After Mr. Netanyahu’s political loss, the White House sees him as wielding less influence over the president’s agenda.

Inside the White House, officials have come to expect forceful rhetoric from Mr. Netanyahu, especially at a high-profile platform such as the U.N.

While the Israeli leader’s public denunciations have long irked Mr. Obama’s advisers, the president’s political victory on the Iran deal has blunted the impact of Mr. Netanyahu’s criticism.

“The president is proud of the strong relationship between our two countries, and the unshakable bond when it comes to our commitment to Israel’s security,” White House press secretary Josh Earnest said on Thursday, when asked about the address.

Although as critical as ever, Mr. Netanyahu appeared to be careful in the address to maintain his ties with the U.S., something Israeli voters value.

His acknowledgment that the deal constrains Iran’s program, even if only for 10 to 15 years, indicates his desire to “change the channel” on the adversarial nature of his relationship with Mr. Obama, said Aaron David Miller, a former Middle East negotiator now at the Wilson Center.

“I suspect the psychological adjustment means that he’s accepted the fact that, if this was a win-lose situation, the president has won,” said Mr. Miller. “He is now forced to acquiesce in the implementation of the accord.”

Tamara Cofman Wittes, a former State Department official in the Obama administration now at the Brookings Institution, said despite tough words about the Iran deal, Mr. Netanyahu emphasized common ground with Mr. Obama in addressing Iran’s destabilizing behavior and enforcing the deal. This acceptance could help move efforts to boost security ties forward, she said.

“There are a number of agenda items for the U.S. and Israel that were kind of on hold because Netanyahu was so invested in all out opposition to the nuclear deal,” Ms. Wittes said.

As part of the White House effort to reduce tensions, administration officials have intensified outreach to American Jewish leaders and groups supportive of Israel, and Mr. Obama plans to meet with Mr. Netanyahu in Washington in October.

A senior administration official on Thursday said the White House hopes the meeting will close the chapter on several years of toxic relations between the two leaders.

Still, Mr. Netanyahu’s supporters don’t see the debate over the nuclear agreement as over. U.S. lawmakers are drafting new legislation to target Iran’s support for international militant groups and to limit the ability of foreign companies to invest in Iran.

Much of the congressional debate over Iran is focused on trying to dry up funding for Iran’s elite military unit, the Islamic Revolutionary Guard Corps, which dominates the Iranian economy.

Israeli officials and many Iran watchers in Washington say they believe there is a high likelihood that Iran will cheat on the nuclear agreement, and that the West must be prepared to move quickly with punishments if it does.

“The next phase is not just enforcing the nuclear accord, strictly, but putting in place new mechanisms to guard against the cancer of Iran spreading across the region,” said Josh Block, president of The Israeli Project, which campaigned against the nuclear deal.

Arab officials also have sharply criticized Iran during the General Assembly, including diplomats from Saudi Arabia and Yemen. Saudi Arabia’s foreign minister, Adel al-Jubeir, publicly campaigned against Russian involvement in Syria, in part, because Moscow is now aligned with Tehran.

“Iran is an occupying force in Syria,” Mr. al-Jubeir said on Tuesday.

The agreement between Iran and the six world powers—the U.S., U.K., Germany, France, China, and Russia—will gradually lift U.N. and U.S. sanctions on Iran in return for stringent controls on Iran’s nuclear program to keep it from developing a nuclear weapon.

Mr. Netanyahu took issue with Obama administration arguments that the deal will prevent Iran from developing nuclear weapons and vowed to take action if necessary.

“I know that preventing Iran from developing nuclear weapons remains the official policy of the international community,” he said in his address. “But no one should question Israel’s determination to defend itself against those who seek our destruction.”

With eventual sanctions relief, Iran will increase its war chest to supply militant groups in the region, Mr. Netanyahu said. Mr. Obama has acknowledged that this could be a drawback of the deal, but said it was an unfortunate consequence of getting Iran to place those constraints on its program.

Mr. Netanyahu said differences with Mr. Obama were “within the family,” praising the U.S. as Israel’s most steadfast ally.

The prime minister’s speech didn’t sway his critics in Israel who have often heard him playing up the Iran threat, said Yehudit Auerbach, a political scientist at the Tel Aviv-based Bar Ilan University.

“Bibi haters won’t become Bibi lovers,” she said.

Ms. Auerbach said that while the nuclear deal had ushered in a new status quo for countries in Europe and the Middle East that were trying to build commercial and economic ties with Iran, Mr. Netanyahu wasn’t backing off efforts to frame Iran as a danger to Israel and the world.

“Bibi says ‘beware,’ but I don’t think people will listen,” she added. “The deal is done.”

—Felicia Schwartz at the United Nations and Rory Jones in Jerusalem contributed to this article.