US President George W. Bush on Friday called Iran a "grave national security concern," but said he was seeking a diplomatic way to cap its nuclear goals.
A hard-line cleric said in Tehran earlier that Bush was using the nuclear issue to further his goal of overthrowing Iran.
At the UN, the five permanent members of the Security Council ended a meeting on how to persuade Iran to suspend uranium enrichment-related activities, but there was no sign they had agreed on a statement.
The Security Council has begun to tackle Iran's case after the UN nuclear watchdog sent the 15 members a report on Wednesday saying it could not verify that Iran's atomic activities were peaceful.
Bush said US concerns were the result of Iran's stated desire to destroy Israel and Washington's belief that Tehran wants to build nuclear bombs -- something the Iranians deny.
"You begin to see an issue of grave national security concern," Bush told a newspaper group.
"Therefore it's very important for the United States to continue to work with others to solve these issues diplomatically, deal with these threats today," he said.
"Bush talks of regime change or change of its behavior, which is the same. It means no Islamic regime," said senior cleric Ahmad Khatami in a sermon.
Meanwhile EU foreign policy chief Javier Solana spoke for the first time publicly of possible sanctions against Iran.
Asked if EU foreign ministers meeting in Salzburg, Austria, would discuss the issue, Solana told reporters: "No. We are talking about a gradual approach to give some room still for diplomacy."
Bush said he assumed the threat was related to the US need for imported energy resources. "For national security purposes we have got to become ... not addicted to oil," he added.
Meanwhile, the five veto-holding members of the UN Security Council considered proposals to pressure Iran to resolve questions about its nuclear program, including demands that it abandon uranium enrichment and stop construction on a reactor, diplomats said.