Secretary of Defense Donald H. Rumsfeld said Tuesday he would not engage in "fantasy land" speculation about a possible U.S. attack on Iran, though he said the Bush administration is concerned about Tehran's nuclear ambitions.

Rumsfeld declined to comment on Iran's claim that it has successfully enriched uranium for the first time.

"I'd rather wait and see what our experts say about it," the defense secretary told reporters shortly after the announcement from Tehran.

The U.N. Security Council has demanded that Iran stop all uranium enrichment activity by April 28. Iran has rejected the demand, saying it has a right to develop the process.

Rumsfeld's comments came a day after President Bush said that force is not necessarily required to stop Iran from having a nuclear weapon. Bush dismissed reports of plans for a military attack against Tehran as "wild speculation."

"The United States of America is on a diplomatic track," Rumsfeld said. "There is obviously concern about Iran. Iran is a country that supports terrorism. It is a country that has indicated" a desire to obtain nuclear technology. "But it is simply not useful to get into fantasy land," Rumsfeld said.

Neither Rumsfeld nor Bush has taken military off the table as a potential option.

Former President Hashemi Rafsanjani said Tuesday that Tehran has successfully enriched uranium for the first time, a major development in its quest to develop nuclear fuel.

Current President Mahmoud Ahmadinejad added that the country "will soon join the club of countries with nuclear technology."

The White House denounced the latest comments from Iranian officials, with press secretary Scott McClellan saying they "continue to show that Iran is moving in the wrong direction."

Claims that Iran has successfully enriched uranium "only further isolate" the regime in Tehran and underscore why the international community must continue to raise concerns about its suspected ambition to develop nuclear weapons, McClellan told reporters traveling on Air Force One with Bush on Tuesday to Missouri.

McClellan noted that the Security Council clock is now running on Iran.

"This is a regime that needs to be building confidence with the international community," McClellan said. "Instead, they're moving in the wrong direction."