Rick Gladstone / New York Times

Efforts led by the United States and Israel to isolate Iran suffered a setback on Wednesday when the United Nations announced that Ban Ki-moon, the secretary-general, would join officials from 120 countries in Tehran next week for a summit meeting that Iran has trumpeted as a vindication of its defiance and enduring importance in world affairs.

Ban’s decision to attend the meeting of the Nonaligned Movement, announced by his spokesman, Martin Nesirky, came despite objections from both the Americans and Israelis, including a phone call from Prime Minister Benjamin Netanyahu of Israel. The decision was announced a few days after the new president of Egypt, a country that has long been estranged from Iran, said he would attend the summit meeting as well, a decision that had already unsettled the Israelis.

Taken together, the moves reinforced Iran’s contention that a reordering of powers is under way in the Middle East, where Western influence is waning, and that the U.S.-Israeli campaign to vilify Iran as a rogue state that exports terrorism and secretly covets nuclear weapons is not resonating in much of the world.

The meeting of the Nonaligned Movement, a group formed during the Cold War, includes a number of other countries that the U.S. has sought to marginalize, among them North Korea and Sudan, whose president, Omar Hassan al-Bashir, is wanted under a war crimes indictment by the International Criminal Court. Although Iran’s hosting of the meeting is strictly a coincidence of history — under a rotating system, Iran presides over the group through 2014 — Iranian leaders have portrayed it as a privilege that repudiates the U.S. narrative.

“The extraordinary effort that the Iranian leaders have put into the summit is intended to showcase Iran’s global role and offer concrete evidence that the U.S. policy of isolating Iran has failed,” said Farideh Farhi, an independent Iranian scholar at the University of Hawaii.

“A case is being made that it is not the ‘global community’ that has problems with the Islamic republic as repeatedly asserted by U.S. officials but merely a U.S.-led-and-pressured coalition of countries,” she said. “And ironically the Obama administration is conceding the point by trying to pressure various leaders from attending the meeting.”

Ban’s decision to participate, which might have gone nearly unnoticed in other years, was particularly fraught now because of the tensions surrounding the host country. Iran has defied U.N. Security Council resolutions to halt its uranium enrichment and has strongly supported the Syrian government’s harsh repression of an armed uprising, a crackdown that Ban has repeatedly condemned.

Ban has also castigated the anti-Semitic statements and calls for Israel’s destruction made recently by Iranian leaders, reminding them that the U.N. Charter prohibits one member from threatening the existence of another.

But many diplomats and others said it would have been extraordinarily difficult for Ban not to go. The 120 countries that are in the Nonaligned Movement represent the biggest single voting bloc in the 193-member General Assembly at the U.N. It is customary for the secretary-general to attend the movement’s annual meetings regardless of political sensitivities surrounding the host country.

“A sizable chunk if not a majority of the world’s population are citizens of nonaligned nations,” said Karim Sadjadpour, an Iran expert at the Carnegie Endowment for International Peace in Washington. “It’s not something the United Nations secretary-general can easily dismiss.”

Acknowledging that Ban has been under pressure not to attend, Nesirky said Ban viewed the visit as a chance to raise all of these issues directly with his Iranian hosts.

“The secretary-general is fully aware of the sensitivities of this visit,” Nesirky told reporters at the U.N. “He’s heard the views of some of those who said he should not go. At the same time, the secretary-general has responsibilities that he is determined to carry out.”

Nesirky also said Ban expected to meet with senior Iranian leaders, including Ayatollah Ali Khamenei. “It is certainly the secretary-general’s expectation that he will have meaningful and fruitful discussions with the supreme leader,” Nesirky said. To boycott the invitation from Iran, Nesirky said, “would be a missed opportunity.”

There was no immediate reaction to Ban’s decision from Israel. But according to Netanyahu’s office, he had telephoned Ban on Aug. 10 and told him that such a trip, even if well-intentioned, would be a mistake. “Your visit will grant legitimacy to a regime that is the greatest threat to world peace and security,” Netanyahu was quoted as saying.

Even before Ban made his decision known, the Israeli government was asserting that the sanctions effort against Iran was not working, a conclusion that was reinforced for the Israelis because of the decision to attend the summit meeting in Iran by President Mohammed Morsi of Egypt.

“If you’re going there, if you’re paying homage to the leaders of Iran, what kind of diplomatic isolation is that?” Mark Regev, Netanyahu’s spokesman, said of Morsi’s decision.

The reaction to Ban’s announcement was more muted from the Obama administration, which had engaged in a less public effort to dissuade him.

Some administration officials sought to put the best face on the situation, urging Ban to exploit the moment to convey his unhappiness with Iran’s behavior.

“We think that Iran is going to try to use the event for propaganda purposes and to try to cover up the extreme isolation Iran is feeling politically and economically,” said Tommy Vietor, the spokesman for the National Security Council. “That said, if people choose to participate, we believe they should take the opportunity of any meetings that they have with Iran’s leaders to press them to comply with their international obligations without further delay.”

The American Jewish Committee, among a number of pro-Israel voices in the U.S. that had exhorted Ban not to visit Iran, called the decision “a grave mistake” in a statement posted on its website.

“Tehran is not the place for the U.N. secretary-general to visit, not at this time, not to meet with this Iranian regime,” David Harris, the group’s executive director, said in the statement. “We are stunned that Secretary-General Ban Ki-moon would honor a regime that consistently ignores both him and the world body he heads in ways that threaten regional and global security.”

Some said that Ban’s three-day visit, which begins next Wednesday, could also turn out badly for Iranian leaders, particularly if he raises issues in an unfiltered way to the Iranian public about the government’s human rights record.

Others said that Ban could surprise critics by confronting or embarrassing Ayatollah Khamenei and his subordinates over their anti-Semitic statements.

“The fact that he’s going is going to be viewed as a victory for Iran,” said Trita Parsi, the president of the National Iranian American Council, an advocacy group of Americans of Iranian descent. “But if pressure leads Ban Ki-moon to express harsh criticism of their statements on Israel, then it could be viewed as a victory for those who had not wanted him to go.”