"Why should Iran suspend its research activities?" Ali Larijani, the head of the Supreme National Security Council, was quoted as saying by the official IRNA news agency.
Enrichment can be extended from making reactor fuel to the production of warheads, but Mr Larijani branded a UN Security Council demand for a suspension by April 28 as "not rational".
"One should not follow such propositions... which are not rational," he said, adding: "Iran will follow its nuclear programme with patience."
Last Tuesday, Iran announced it had successfully enriched uranium to the level needed for reactor fuel, reigniting fears that the hardline regime would soon acquire the technical know-how to make bombs.
The deputy head of Iran's Atomic Energy Organisation, Mohammad Saidi, argued that the UN nuclear watchdog had failed to find any proof that Iran's program was anything other than a legitimate effort to generate electricity.
"Therefore, there is no need to continue a suspension," he told the
Etemad-Melli newspaper. "These countries have to accept the reality and realize they are talking with a country that masters this technology."
The five permanent members of the Security Council, Britain, China, France, Russia and the United States, plus Germany will meet in Moscow Tuesday to discuss the issue amid a US push for robust UN action.
Iranian official in Washington
In a minor embarrassment for the US the State Department has confirmed a senior official from Iran was in Washington.
But a State Department would not say how he got into the country or what he was doing here.
Department spokesman Sean McCormack said Mohammad Nahavandian was in Washington but added, "He's not here for meetings with US government officials to my knowledge; certainly not with members of the State Department."
Mr McCormack said Mr Nahavandian had not been issued a visa but was in the United States legally. He did not elaborate but said only, "There are a variety of other ways for an individual to arrive in the country."
The Washington visit by Nahavandian, described as an economic aide to Iran's top nuclear negotiator Ali Larijani, was first reported 10 days ago by Britain's Financial Times newspaper.
The Financial Times quoted an Iranian adviser as saying that Mr Nahavandian had gone there to discuss the possibility of wide-ranging direct talks between the two countries, which have not had diplomatic relations for a quarter-century.
Iran attack plan
Meanwhile the Pentagon has declined to comment on a report that United States military planning for Iran began in 2002 and has been continually updated since.
"This is the United States Defence Department. We plan for all sorts of things," said Bryan Whitman, a Pentagon spokesman.
"With respect to Iran the United States government approach has been very clear," he added, saying it was working through the international community to try to resolve concerns about Iran's nuclear program diplomatically.
William Arkin, a well-connected military analyst writing in the Washington Post, said the planning had been conducted under the codename TIRANNT, an acronym for Theatre Iran Near Term.
It includes a scenario for a land invasion led by the US Marine Corps, a detailed analysis of the Iranian missile force and a global strike plan against any Iranian weapons of mass destruction, Mr Arkin wrote.
Air force planners have modelled attacks against Iranian air defences, while navy planners have evaluated coastal targets and drawn up scenarios for keeping control of the Strait of Hormuz.