Iran has again insisted it will keep enriching uranium in spite of growing international concern that it is pursuing nuclear weapons.
Iran has refused to halt work on uranium enrichment
Ali Larijani, its top nuclear official, said demands to halt the programme were "irrational" and he advised other states "not to repeat past mistakes".
US senators have called for direct talks between America and Iran.
New satellite photographs published in the US appear to show Iran has expanded and reinforced its main nuclear plants.
Uranium conversion facilities at Isfahan look to have been expanded while an underground uranium enrichment plant at Natanz appears to have been reinforced, the US-based Institute for Science and International Security (Isis) think-tank reports.
According to Isis, Natanz's two subterranean cascade halls have been buried by successive layers of earth and concrete and the roof of the halls is now eight metres (26 feet) underground.
"Iran is taking extraordinary precautions to try to protect its nuclear assets," David Albright, the ex-UN arms inspector who heads Isis, told Reuters news agency.
War of words
"Iran will follow its nuclear programme with patience," Mr Larijani was quoted as saying by state news agency Irna on Monday.
US calls for the UN to authorise more robust action against Iran - including the possible use of force - were, he said, not new and would not affect Iran's determination.
Akbar Hashemi Rafsanjani, the former Iranian president, has predicted the US will never attempt to attack his country because of the risks involved.
On a visit to Kuwait, he told its parliament: "Reports about plans for an American attack on Iran are incorrect... The consequences would be too dangerous."
Iran, he added, was certain Gulf countries would not assist the US in any attack.
Last week, President Iranian Mahmoud Ahmadinejad announced Iran had successfully produced enriched uranium but insisted it did not want nuclear weapons.
He later made a new prediction that Israel would be destroyed, saying it would "fall with a storm".
His words were dismissed by Israeli veteran statesman Shimon Peres, who said Mr Ahmadinejad would "end up like Saddam Hussein", ousted by the US invasion of Iraq in 2003.
Richard Lugar, Republican chairman of the Senate foreign relations committee, called for direct talks between Washington and Tehran.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is chemically processed and converted into a gas by heating it to above 64C (147F)
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
Despite being a senior member of the same party as President George W Bush, Senator Lugar's comments fly in the face of the policy of the Bush administration.
It is following a multi-lateral approach through the UN Security Council.
The Council is due to discuss Iran on 28 April - the deadline given to Iran to address its concerns.
Sen Lugar also urged caution on sanctions while Secretary of State Condoleezza Rice was already outlining sanctions last week.
Pope Benedict XVI has called for a negotiated solution to the crisis.
In his traditional Easter message in St Peter's Square in Rome, he urged "an honourable solution... for all parties, through honest and serious negotiations".