Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, also acknowledged differences in perspective between the United States and Israel over the best way to handle Iran and its nuclear program.

He said the United States was convinced that sanctions and diplomatic pressure was the right path to take on Iran, along with "the stated intent not to take any options off the table."

"I'm not sure the Israelis share our assessment of that. And because they don't and because to them this is an existential threat, I think probably that it's fair to say that our expectations are different right now," Dempsey said in an interview as he flew to Washington from London.

Asked whether he was talking about the differences between Israeli and US expectations over sanctions, or differences in perspective about the future course of events, Dempsey said: "All of the above."

He also did not disclose whether he believed Israel was prepared to strike Iran.

European Union foreign ministers slapped sanctions on more than 180 Iranian companies and individuals Thursday over Tehran's refusal to meet international demands to halt sensitive nuclear work. The ministers also agreed to continue working on extra punitive measures that could target Iran's vital energy sector, a diplomat said.

There has been concern that if world powers cannot nudge Iran into serious nuclear negotiations, then Israel, which feels threatened by Iranian nuclear aspirations, will attack.

Asked directly whether Israel would alert the United States ahead of time if it chose to go forward with military action, Dempsey replied flatly: "I don't know."

Last week, US Defense Secretary Leon Panetta raised American concerns about the unintended consequences of any military action against Iran during talks with his Israeli counterpart, Ehud Barak, at a security forum in Canada.

Those included US fears about fallout on the world economy and that a strike would only delay — not derail — Iran's nuclear program.