DAVOS, Switzerland — The head of the U.N. atomic watchdog agency, in an indirect warning to the United States and Israel, said Thursday a military strike on Iran’s nuclear facilities would have “catastrophic” consequences and only strengthen Tehran’s resolve to make atomic arms.

Mohamed ElBaradei, director general of the International Atomic Energy Agency, also cited Western intelligence assessments that Iran may be only four years away from having the capacity to produce such weapons. But he stressed that his agency’s inspectors had turned up no firm evidence of such intentions.

Still, he indicated there are some in Tehran who favored developing such arms.

“A preventive strike would be catastrophic,” ElBaradei said at a panel discussion at the World Economic Forum, adding it would only “strengthen the hand of those who say ‘let us develop a weapon.”

While ElBaradei did not name any nation, his comments were clearly directed at Israel and the United States, which have both suggested a strike on Tehran’s nuclear facilities was not off the table unless the Islamic republic ended its nuclear defiance.

In a clear warning of such possible intentions, Israeli Prime Minister Ehud Olmert said Wednesday his country — believed now to be the only nuclear-armed state in the Middle East — will respond to the threat posed by Iran’s nuclear ambitions “with all the means at our disposal.”

It also came as the United States beefed up its naval presence in the Gulf, sending a second U.S. aircraft carrier group there to signal Iran it will not tolerate any attempts to dominate the region.

The U.N. Security Council imposed sanctions on Iran last month for defying demands to freeze uranium enrichment. Tehran says it wants to perfect the process to generate electricity, but its other use — creating the fissile core of nuclear weapons — has fed international concerns about Iran’s true intentions.

Instead of compromise, Iran has ramped up the rhetoric and its nuclear activity.

Iranian President Mahmoud Ahmadinejad said Tuesday that Washington was incapable of inflicting “serious damage” on his country. And U.N. officials told the Associated Press last week that Iran was ready to start assembling thousands of centrifuges to enrich uranium after finishing work on an underground facility housing such machines.

Even if Tehran proves successful in installing 3,000 centrifuges — in the first stage of what it says will be a network of more than 50,000 such machines — experts estimate it would take several years for all of them to be running smoothly. Once that happens, Tehran could produce two bombs a year.

“They have the knowledge, sure they have the knowledge,” said ElBaradei of Iran’s nuclear program — which has been under IAEA investigation for more than four years. “Are you going to bomb the knowledge?”

ElBaradei indicated he was not against U.N.-sanctioned force against world renegades as a last option. But “in the case of Iran, we are absolutely far away from it.”

Talks, first between Tehran and European powers Britain, France and Germany and then the five Security Council permanent members and Germany have failed over more than two years to persuade the Islamic republic to shelve enrichment plans and led to the U.N. sanctions.

But ElBaradei said new negotiations — this time involving not only the great powers but all countries in the region — were the best way to reach compromise. And the “U.S. has to be engaged,” he said, in indirect criticism of America’s refusal up to now to hold one-on-one talks with Iran.