President Bush said Tuesday a nuclear-armed Iran would blackmail the free world and raise a mortal threat to the American people.

"I am not going to allow this to happen," Bush said in a speech on terrorism. "And no future American president can allow it, either."

With allies urging a go-slow approach on pressuring Iran to stop enriching uranium, Bush pledged to work closely with other governments to find a diplomatic solution to the standoff with Tehran over its nuclear programs.

But he also stressed, "the world's free nations will not allow Iran to develop a nuclear weapon."

Iran has refused to stop enrichment as a precondition to negotiations on U.S. and European offers of concessions, including U.S. help with civilian nuclear programs.

"Their choice is increasingly isolating the great Iranian nation from the international community," Bush said.

"It is time for Iran's leader to make a different choice," he said.

While standing firm on seeking U.N. sanctions against Iran, the administration acknowledged Tuesday there would be weeks of intensive diplomacy to nail them down.

"There is going to be some work that's required in the Security Council," State Department spokesman Sean McCormack said. "I would expect that that is going to be tough, intensive diplomacy over the course of the coming weeks."

But he said the U.N. Security Council had already made it clear in a resolution that if Iran failed to meet an Aug. 31 deadline to suspend uranium enrichment, the Council was prepared to vote for sanctions.

And so, he said, the United States intends to proceed "down that pathway."

Iran has renewed to U.N. Secretary-General Kofi Annan an offer of negotiations about its nuclear programs, but has declined to suspend enrichment of uranium as a precondition.

Tehran insists that its nuclear activities are designed to produce civilian power and are within its rights.

Undersecretary of State Nicholas Burns is due to hold talks in Berlin on Thursday with European, Russian and Chinese diplomats on strategy for applying sanctions.

To maximize chances for agreement, the Bush administration is inclined to seek graduated sanctions, beginning with such limited measures as restrictions on providing Iran with technology that could be used in military programs.

European governments as well as Russia and China have urged patience with Iran. The senior European diplomat, Javier Solana, is expected to meet Wednesday in Vienna with Ali Larijani, the chief Iranian nuclear negotiator.

McCormack said the administration would encourage governments "that might have some sway over Iran to engage them and to send a very clear message to them that they need to comply with the just demands of the international community."