Iran has ignored a UN Security Council call to suspend all nuclear fuel enrichment, a report by the head of the UN's nuclear watchdog has concluded.
Mr Ahmadinejad said Iranians should not be denied their rights
Mohamed ElBaradei's report said that Tehran had done little to answer questions over its nuclear intentions.
The UN wants the research halted amid Western fears - denied by Iran - that Tehran might develop nuclear weapons.
US Ambassador John Bolton said he would now push for Iran to face mandatory UN demands to stop its nuclear work.
President Mahmoud Ahmadinejad has said Iran does not "give a damn" about UN resolutions over its research.
Mr ElBaradei, chief of the International Atomic Energy Agency (IAEA), told the Security Council that Iran had not halted its programme and had not given information on key issues.
IRAN CRISIS: NEXT STEPS
2 May: Negotiators from US, Russia, China, UK, France and Germany meet in Paris
3 May: Possible Security Council meeting to discuss IAEA report
9 May: Foreign ministers from US, Russia, China, UK, France and Germany meet at UN
This was in defiance of a Security Council warning, which gave Iran until 28 April to comply.
"After more than three years of agency efforts to seek clarity about all aspects of Iran's nuclear programme, the existing gaps in knowledge continue to be a matter of concern," his report said.
"Any progress in that regard requires full transparency and active co-operation by Iran."
Among the report's findings:
- Iran's claim to have enriched low-grade uranium appears true
- Iran used undeclared plutonium in small-scale separation experiments
- Iran refused to give more information on key issues, including centrifuge programmes
The deputy head of Iran's Atomic Energy Organisation, Mohammad Saeedi, said the IAEA report was generally acceptable.
"The report does not contain negative points. It shows that the agency still has the capacity to review Iran's nuclear case," he said, adding that Iran was ready to answer the remaining issues.
The US Ambassador to the UN, John Bolton, said it was clear that Iran had accelerated its nuclear programme and that he hoped the Security Council would act as soon as possible.
"We're invoking the mandatory provisions of Chapter VII [of the UN Charter]," Mr Bolton said.
Chapter VII, which is invoked in cases of threat to international peace and security, could open the way to sanctions or even military action, although this step is so far highly unlikely.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is chemically processed and converted into Uranium Hexofluoride gas
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
The BBC News website's world affairs correspondent, Paul Reynolds, says the next move in the Security Council is expected to be an attempt by Western countries to make the UN demands mandatory.
He says this would be in the form of a Security Council resolution, but one that would not threaten sanctions because of opposition from Russia and China.
The stage is set for months of diplomatic confrontation with and over Iran, our correspondent says.
The next immediate steps will be on 2 May when negotiators from the US, Britain, France, Germany China and Russia will meet.
US Secretary of State Condoleezza Rice is to hold talks with foreign ministers of the other four permanent Security Council members and Germany on 9 May.
Iran says it has the right to peaceful nuclear technology and denies Western claims that it is seeking weapons.
Tehran has vowed to hit US targets worldwide if subjected to a US military attack.
Supreme Leader Ayatollah Ali Khamenei also said this week Iran was ready to share its nuclear technology with other nations.