“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.”

Mr. 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 United Nations Security Council resolutions to halt its uranium enrichment and has strongly supported the Syrian government’s sharp repression of an armed uprising, a crackdown that Mr. Ban has repeatedly condemned.

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

But many diplomats and others said it would have been extraordinarily difficult for Mr. 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 United Nations. It is customary for the secretary general to attend the movement’s annual meetings regardless of political delicacies 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 Mr. Ban has been under pressure not to attend, Mr. Nesirky, his spokesman, said Mr. Ban viewed the visit as a chance to raise the issues of Iran’s nuclear program, its support for Syria and its campaign against Israel directly with his hosts.

“The secretary general is fully aware of the sensitivities of this visit,” Mr. Nesirky told reporters at the United Nations. “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.”

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Mr. Nesirky also said Mr. 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,” Mr. Nesirky said. To boycott the invitation from Iran, Mr. Nesirky said, “would be a missed opportunity.”

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There was no immediate reaction to Mr. Ban’s decision from Israel. But according to Mr. Netanyahu’s office, he had telephoned Mr. 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,” Mr. Netanyahu was quoted as saying.

Even before Mr. 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 Mohamed 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, Mr. Netanyahu’s spokesman, said of Mr. Morsi’s decision.

The reaction to Mr. 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 Mr. 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 United States that had exhorted Mr. Ban not to visit Iran, called the decision “a grave mistake” in a statement posted on its Web site.

“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.”

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Some said that Mr. 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 Mr. 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.”