"That is the last option. Everything else has to be exhausted. But to say under no circumstances would we exercise a military option, that would be crazy," Republican Senator John McCain of Arizona said on CBS's Face the Nation
Democratic Senator Evan Bayh of Illinois, a member of the Senate Intelligence Committee, said there are sensitive elements of Iran's nuclear program, which, if attacked, "would dramatically delay it's development".
"But that should not be an option at this point. We ought to use everything else possible keep from getting to that juncture," he said on CNN's Late Edition.
Bayh accused President George Bush of undermining the US national interest and creating this "dilemma" by ignoring the problem of Iran for four years.
A growing nuclear fracas exploded last week when Iran, defying the United States and major European powers, resumed nuclear research after a two year moratorium.
"[T]here is only one thing worse than the United States exercising a military option, that is a nuclear armed Iran"
Republican Senator John McCain
Iran says it aims only to make power for an energy-needy economy, not build atom bombs. But it hid nuclear work from the UN nuclear watchdog agency for almost 20 years before exiled dissidents exposed it in 2002.
On Sunday, Iran said that only diplomacy, not threats to refer it to the UN Security Council, could defuse a standoff over its nuclear work and warned that any Western push for sanctions could jack up world oil prices.
The Security Council's five permanent members and Germany planned talks in London Monday on a common strategy to tackle the controversy.
McCain (r) says Bush has ignored
the Iran threat for too long
McCain called the nuclear standoff "the most grave situation that we have faced since the end of the Cold War, absent the whole war on terror".
"We must go to the UN now for sanctions. If the Russians and the Chinese, for reasons that would be abominable, do not join us then we will have to go with the (states that are) willing," he said.
While acknowledging that President Bush has "no good option", McCain said "there is only one thing worse than the United States exercising a military option, that is a nuclear armed Iran".
"If the price of oil has to go up then that's a consequence we would have to suffer," he said.
Iran is the world's fourth biggest exporter of crude oil and the second biggest in the Organisation of the Petroleum Exporting Countries.
Experts and officials say it may be impossible to destroy Iran's nuclear program because much of it is underground and dispersed at numerous sites.
In addition, they have said an attack on Iran could further inflame anti-Americanism in the Middle East and prompt Tehran to interfere more in Iraq and encourage Islamist fundamentalist groups to launch new attacks on the West.
Another Senate Intelligence Committee member, Republican Trent Lott of Mississippi, said that despite a massive military commitment in Iraq the US has the capability to strike Iran, but it would be "difficult" and other options must be tried first.