Iranian President Mahmoud Ahmadinejad on Sunday ordered the Atomic Energy Organization of Iran (AEOI) to be on standby to start enriching uranium to a purity of 20 percent, infuriating Western countries who want Iran to halt nuclear activities.

Head of the AEOI Ali Akbar Salehi said on Sunday the country was to begin enrichment on Tuesday and would inform the International Atomic Energy Agency (IAEA) of its plan a day before.

Iran, currently enriching uranium to a purity of about 3.5 percent, said it wanted higher-grade uranium to power a research reactor producing medical isotopes. Such a move prompted the Western nations to step up pressure on the Islamic republic, signaling new sanctions.

U.S. Defense Secretary Robert Gates, who was visiting Italy said Iran's response was "very disappointing." He called on the international community to pressure Iran into abandoning its unclear program.

"If the international community will stand together and bring pressure to bear on the Iranian government, I believe there is still time for sanctions and pressure to work," he told a press conference, but didn't give any details on the kinds of sanctions.

Germany also raised sanction threat. Its Defense Minister Karl-Theodor zu Guttenberg said the international community should make it clear to Iran that "patience is at an end," adding "we need to consider very carefully what impact our options could have."

Meanwhile, British Foreign Office said in a statement that Iran's decision to upgrade nuclear fuel was "clearly a matter of serious concern," which would constitute a "deliberate breach" of UN resolutions.

Iran has already had three sets of sanctions imposed on it.

Amid complaints and sanctions threat from Western powers, Iranian President Mahmoud Ahmadinejad said on Sunday his country had not "closed door" to the nuclear swap offer, although it was capable of producing the 20 percent enriched fuel itself.

The IAEA proposed last October that Iran export 1,100 kg of its uranium to Russia and France by the end of last year to turn it into enriched fuel rods that could be used for Iran's research reactor.

Iran, who had refused the proposal said now it was fine to carry out the swap deal if the world powers were willing to cooperate with the country unconditionally.

However, the AEOI head said his country will enrich uranium to a level of 20 percent at its Natanz enrichment facilities if the swap deal fails.

For years, Western countries have suspected that the Islamic republic is enriching uranium to make a nuclear bomb, while Iran denied the accusation saying it has the right to pursue its nuclear program for peaceful use.