NEW YORK — When Iran’s president accused the U.S. at the United Nations General Assembly last year of orchestrating the 9/11 attacks, American diplomats were not caught flat-footed by the tirade.

Even before President Mahmoud Ahmadinejad finished his incendiary rant, U.S. diplomats marched out of the cavernous U.N. hall in protest and were ready with a written statement condemning his comments.

It was as if the U.S. knew exactly what Ahmadinejad intended to say.

The walkout hinted at one of the well-known but seldom spoken truths about the United Nations: The international organization, which was founded in the name of peace and security, is also a hotbed of spying and clandestine operations, where someone might very well be listening to your conversations and monitoring your emails — or perhaps reading your speeches in advance.

The start of the General Assembly each year is the Super Bowl of the U.N. spy games.

Foreign leaders descend upon New York with entourages of aides and security officers. Many have not been dispatched to practice diplomacy. They are intelligence officers, and they’ve come instead to recruit agents in hotels and quiet cafes around the city. In their line of work, trickery and deception trump political niceties.

While the diplomats inside the United Nations are often making headlines, FBI agents are chasing spies around the city. Justice Department lawyers are asking judges to approve wiretaps. And the CIA is searching for foreigners who might be persuaded to commit treason.

All this makes for a frenzied few weeks, especially for the FBI’s Manhattan field office. The FBI’s counterintelligence unit there is responsible for monitoring foreign diplomats in the city.

It’s one of the most sophisticated intelligence-gathering operations in the U.S. and involves one of the FBI’s most extensive electronic surveillance programs, according to former U.S. intelligence officials speaking on the condition of anonymity to discuss sensitive matters.