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Defense Secretary Leon E. Panetta earlier this month said the United States will not tolerate a nuclear-armed Iran.

One analyst suggested that the increased pressure on Iran is politically motivated by the presidential election campaign, especially with Republican candidates charging that Mr. Obama is weak on Iran.

“I would characterize it as buying time until after the elections,” said Reza Marashi, director of research at the National Iranian American Council and a former State Department official.

“No politician, particularly a sitting president … wants to look weak on national security issues on the run up to his or her re-election effort, so that forces [Mr. Obama] to take harder measures on Iran.”