WASHINGTON - The consequences of Iran acquiring a nuclear weapon would be calamitous and major powers must act together to prevent it, the top U.S. military officer said on Thursday.

Admiral Mike Mullen's remarks came the day after Iran's president announced the country had tested a missile that analysts said could hit Israel and U.S. bases in the Gulf, a major source of crude oil for the United States.

The United States and other Western powers are concerned that Iran could combine elements of its uranium enrichment and missile programs to create a nuclear weapon, although Tehran denies it intends to do this.

I'm one who believes that Iran getting a nuclear weapon is calamitous for the region and for the world, Mullen, chairman of the U.S. Joint Chiefs of Staff, told the U.S. Senate Foreign Relations Committee.

It then, in my view, generates neighbors who feel exposed, deficient and then develop or buy the capability themselves, he said, suggesting Iran's acquisition of a nuclear weapon likely would trigger a nuclear arms race in the Middle East.

The downside, potentially, is absolutely disastrous.

But Mullen did not suggest the United States should take military action to prevent Iran from acquiring nuclear weapons.

He echoed the Obama administration's policy that big powers should work together to persuade Iran not to pursue a nuclear bomb and halt the proliferation of nuclear weapons generally.

Major leaders, internationally, have got to come together to arrest this growth or the long-term downside for the people in the world is really, really tragic and drastic, he said.

Israel and the United States have not ruled out military action against Iran but the Obama administration has adopted a policy of trying to engage Tehran diplomatically to resolve differences.

President Barack Obama said on Monday he hoped to begin negotiations with Tehran soon, after Iran holds elections next month. Iran's leaders largely have rebuffed his efforts so far to reach out to them, and toughened their rhetoric.

Obama also set a rough timetable for his diplomatic outreach, saying that by the end of this year the United States should have a sense of whether the effort was making progress.

Mullen has previously expressed concern that a military strike against Iran could add further turmoil to the unstable Middle East.

A U.S. National Intelligence Estimate issued in late 2007 concluded that Iran did have a nuclear weapons program but put the program on hold in 2003.

U.S. officials say they believe Tehran still wants to acquire at least the capability to make a nuclear bomb. Iran says its nuclear program is only to produce electricity so it can export more of its valuable oil and gas.