President Obama said Tuesday that he's "bent over backwards" to engage Iran in "constructive" dialogue and the United States will push the United Nations to sanction the country, which has begun enriching uranium to levels capable of making an atomic bomb.

Obama told reporters during a rare press conference Tuesday that the U.S. is developing a "significant regime of sanctions" in response to the Islamic Republic's move to enrich uranium to 20 percent purity level in defiance of world powers.

"That indicates to us that despite their posturing that their nuclear power is only for, for civilian use that they in fact continue to pursue a course that would lead to weaponization," Obama said. "That is not acceptable to the international community.

Obama said the process toward adding new sanctions is moving along quickly, but he gave no specific timeline.

Iran began enriching uranium to a higher level on Tuesday, raising fears that the process could eventually be used to give the Islamic republic nuclear weapons.

Defense Secretary Robert Gates said the Obama White House has gone further than any administration to reach out to Iran, and called the move "disappointing."  He said Tehran is violating a Nuclear Non-Proliferation Treaty and is refusing to sign off on an IAEA-brokered deal that would allow Tehran to be supplied with nuclear fuel for its medical reactor in exchange for its low-enriched uranium (LEU).

"I have never seen an administration reach out in as principled and comprehensive a way as President Obama has done," Gates said in a press conference Tuesday in Ankara, Turkey. "The response has been quite disappointing."

The Pentagon said the U.S. wants a U.N. Security Council resolution "within weeks" to halt the program, which it claims is intended to make a nuclear bomb.

Iran has defied five U.N. Security Council resolutions -- and three sets of U.N. sanctions -- aimed at pressuring it to freeze enrichment, and has instead steadily expanded its program.

The country denies claims that its uranium enrichment is intended to make a nuclear warhead, saying the program is meant only for peaceful purposes. Iran, which currently enriches uranium to 3.5 percent purity, said it wants to process uranium to 20 percent so it can be used as fuel to power a research reactor in Tehran that makes medical isotopes for cancer patients.

Iranian envoy Ali Asghar Soltanieh told The Associated Press Monday that he informed the International Atomic Energy Agency of the decision to enrich at least some of its low-enriched uranium stockpile to 20 percent -- considered the threshold value for highly enriched uranium. The material in high purity form can be used in the fissile core of a nuclear bomb.

Soltanieh, who represents Iran at the Vienna-based IAEA, also said that the U.N. agency's inspectors are overseeing enrichment to low levels would be able to stay on site to fully monitor the process. And he blamed world powers for Iran's decision, asserting that it was their fault that a plan that foresaw Russian and French involvement in supplying the research reactor had failed.

"We cannot leave hospitals and patients desperately waiting for radio isotopes" being produced at the Tehran reactor and used in cancer treatment, he said.

Soltanieh declined to say how much of Iran's stockpile -- now estimated at 1.8 tons -- would be enriched.

In an interview with Fox News on Tuesday, Assistant Secretary of State P.J. Crowley called the decision a "provocative act in defiance of U.N. Security Council resolutions," saying it "does not meet the humanitarian needs of the Iranian people and it risks creating more regional instability."

"Iran can't fabricate the fuel for the Tehran Research Reactor in time to ensure and uninterrupted supply so we offered to exchange their fuel for suitable fuel for the TRR, the Tehran research reactor," Crowley said.

"That has been a deal they have been unwilling to accept," he said, adding that this "calls into question what their real motivations are."

Iran's intention to increase its nuclear enrichment program came just days after Ahmadinejad appeared to move close to endorsing the original deal, which foresaw Tehran exporting the bulk of its low-enriched uranium to Russia for further enrichment and then conversion for fuel rods for the research reactor.

The original plan was welcomed internationally because it would have delayed Iran's ability to make a nuclear weapons by shipping out about 70 percent of its low-enriched uranium stockpile, thereby leaving it with not enough to make a bomb. Tehran denies nuclear weapons ambitions, insisting it needs to enrich to create fuel for an envisaged nuclear reactor network.

Crowley called the IAEA-brokered proposal the "most practical and responsible solution."

"It's still on the table but they haven't said 'yes.'"

Fox News' Justin Fishel and NewsCORE and The Associated Press contributed to this report.