Anthony Zinni, a retired general and former head of US Central Command, told CNN on Sunday that a pre-emptive strike on Iran would be extremely risky
"Any military plan involving Iran is going to be very difficult. We should not fool ourselves to think it will just be a strike and then it will be over," said Zinni.
"The Iranians will retaliate, and they have many possibilities in an area where there are many vulnerabilities, from our troop positions to the oil and gas in the region that can be interrupted, to attacks on Israel, to the conduct of terrorism."
But he said he had no detailed knowledge of the alleged military plans.
Height of irresponsibility
John Kerry, a Democratic senator and former presidential contender, also assailed the White House for what he said was its over reliance on military might.
"That is another example of the shoot-from-the-hip, cowboy diplomacy of this administration.
"The Iranians will retaliate, and they have many possibilities in an area where there are many vulnerabilities, from our troop positions to the oil and gas in the region that can be interrupted, to attacks on Israel, to the conduct of terrorism"
Anthony Zinni, former head of US Central Command
"For us to think about exploding tactical nuclear weapons in some way is the height of irresponsibility. It would be destructive to any non-proliferation efforts and the military assessment is: It would not work," he told NBC television.
Both men made their remarks after the publication of two media reports this weekend that said George Bush, the US president, was seriously considering military action against Iran, including using nuclear weapons, amid a stalemate in diplomatic efforts.
But the Bush administration said on Sunday that its priority was to seek a diplomatic solution to the dispute over Iran's nuclear ambitions.
A senior official labelled the reports "ill-informed", although he stopped short of an outright denial.
"The president's priority is to find a diplomatic solution to a problem the entire world recognises," the official said.
"And those who are drawing broad, definitive conclusions based on normal defence and intelligence planning are ill-informed and are not knowledgeable of the administration's thinking on Iran."
Jack Straw, the British foreign secretary, on Sunday rejected the idea that Washington could launch a military strike on Iran, saying it was "not on the agenda" and that any idea that Washington could use nuclear weapons against Iran was "completely nuts".
Investigative journalist Seymour Hersh wrote in the New Yorker magazine that Washington was stepping up planning for a possible bombing campaign against Iran, despite publicly pushing for a negotiated settlement.
He said that one option being considered called for the use of nuclear "bunker-busting" bombs against Iranian targets. He also asserted that Washington already had advance forces on the ground in Iran.
The Washington Post and the Times of London, citing unnamed US officials and independent analysts, also reported that the administration was studying options for strikes against Iran.
Iran insists it wants nuclear technology only for power generation. Washington says Tehran is trying to build an atomic bomb and refuses to rule out military options to deal with what it says is one of the world's biggest threats.
Appeals by the US - already bogged down in Iraq - for sanctions on Iran have been frustrated by the reluctance of fellow UN Security Council veto-holders Russia and China to take such action.