The Iranian leader told a news conference that the new facility won't be operational for 18 months so Iran has not violated any requirements of the International Atomic Energy Agency.
"What we did was completely legal, according to the law. We have informed the agency, the agency will come and take a look and produce a report and it's nothing new," he said.
U.S. President Barack Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy accused Iran on Friday of constructing a secret underground uranium enrichment facility and of hiding its existence from international inspectors for years. The charges came at a meeting of the Group of 20 economic powers in Pittsburgh, and a week before direct talks with Tehran over its nuclear program.
U.S. intelligence has been monitoring construction of the facility for years, reports CBS News chief White House correspondent Chip Reid. Bush administration officials briefed then President-elect Obama during the transition.
His administration continued the Bush policy of keeping it secret while intelligence was gathered to make an air-tight case that the purpose was nuclear weapons, not nuclear power. The president decided to go public today after it became clear Iran had learned U.S. intelligence was on to them, Reid reports.
The disclosure puts heavy new pressure on Tehran to quickly disclose all its nuclear efforts - including any moves toward weapons development - "or be held accountable." (.)
Iran acknowledged the facility's existence for the first time in a letter Monday to the Vienna-based IAEA that said the enrichment level would be up to 5 percent, suitable only for peaceful purposes. Weapons-grade material is more than 90 percent enriched.
But Ahmadinejad contended just hours later that, "Iran's activities with respect to the peaceful use of nuclear technology are completely within the framework of IAEA rules and under its supervision."
He contended that enrichment facilities need not be disclosed until six months "before it is infused with gas" and operations begin.
The International Atomic Energy Agency, however, has rejected Iran's contention it must notify the agency of new facilities only six months before operations. The agency says Iran is obliged to make such a notification when it begins design of such facilities.
The Iranians said in March 2007 they were "suspending" the modification to their IAEA safeguards agreement requiring that early notification. But the IAEA countered that a government cannot unilaterally abandon such an agreement.
In countering the charges that the underground facility near the Shiite Islam holy city of Qom, about 100 miles southwest of Tehran, was being hidden, Ahmadinejad said, "we actually informed the agency 18 months ahead of time," he insisted, adding that Obama would regret the statement.
"If I were [President] Obama's adviser, I would definitely advise him to refrain making this statement because it is definitely a mistake. It would definitively be a mistake," Ahmadinejad told TIME Magazine.
"I'm sure they'll definitely feel sorry about it. I think they probably already regret it and will be regretting it more down the road," Ahmadinejad said. "At the end of the day, this is a very ordinary facility that has been set up, and it's only in the beginning stages."
Speaking at an overflowing news conference, Ahmadinejad dodged a question about whether Iran had sufficient enriched uranium to manufacture a nuclear weapon, but said Tehran rejects such armaments as "inhumane."
"We believe that nuclear weapons are against humanity," he said. "This bomb is retarded politically. ... This bomb belongs to the last century."
A senior U.S. official who is familiar with intelligence on the secret Iranian uranium-enrichment plant described it as "heavily protected and heavily disguised," said CBS News White House correspondent Peter Maer.
The official told CBS News the U.S. has been aware of the facility for "several years," and wanted to use the intelligence for "building a case" against Iran.
The intelligence might have stayed quiet, CBS News correspondent Peter Maer reports, if the Iranians had not learned that its secret was "compromised," discovering "fairly recently" that the U.S. was on to the plant's existence, according to the official.
Iran revealed the existence of a second plant in a letter sent Monday to IAEA chief Mohamed ElBaradei - a letter Maer said was characterized by one official as "vague."
Ahmadinejad took a softer tone on many matters while in New York for the U.N. meetings, emphasizing his interest in improving relations with the United States and expressing an openness to include nuclear matters on the negotiations agenda.
He gave no sign, however, that his country was willing to bargain away its nuclear program, which he insists is for peaceful purposes only.
"We have not actually changed our mind," Ahmadinejad told CBS Evening News anchor Katie Couric in an exclusive interview before his address to the U.N. "Our nuclear program will be pursued in accordance to international law."
On Wednesday night, Ahmadinejad addressed the U.N. General Assembly and said Tehran was ready to meet conciliation with conciliation. The Iranian leader issued stinging attacks on the United States and its allies without calling them by name and laced his speech with anti-Israeli and anti-Semitic remarks, prompting a walkout by the U.S. delegation.
The Security Council has imposed three rounds of sanctions to pressure Iran to suspend its uranium enrichment program and start negotiations.
Foreign ministers of six global powers dealing with Iran's nuclear program also met on the sidelines of the General Assembly on Wednesday. They said that they expect Tehran to come clean about its nuclear program at the Oct. 1 talks in Geneva.