Iran will not go back on its controversial nuclear programme, President Mahmoud Ahmadinejad says.
President Ahmadinejad was addressing a gathering of clerics
A day before leading UN members discuss further sanctions against Tehran, he compared Iran's programme to a train with no brakes and no reverse gear.
The president's tone was echoed by a deputy foreign minister who said Iran was ready for any situation, even war.
But US Secretary of State Condoleezza Rice said what Iran needed was not a reverse gear, but a stop button.
She also said she was prepared to meet Iranian officials if Iran stopped nuclear enrichment.
Earlier US Vice-President Dick Cheney renewed a warning that the use of force could be an option if Iran continues to defy the West.
Meanwhile, foreign ministers from seven states with majority Muslim populations met in Pakistan and called for a diplomatic solution to the "dangerous" stand-off.
"It is vital that all issues must be resolved through diplomacy and there must be no resort to use of force," said a statement issued after talks involving ministers from Egypt, Indonesia, Jordan, Malaysia, Pakistan, Saudi Arabia and Turkey.
Permanent members of the UN Security Council and Germany are to meet in London on Monday to discuss further sanctions against Iran.
POSSIBLE NEXT STEPS
New UN resolution on tougher economic sanctions, tabled by US or European allies
US pressure on Europeans to step up bilateral sanctions
New initiative to get Iran back to talks
The meeting was announced after the UN nuclear agency watchdog confirmed Iran had ignored a deadline to suspend its nuclear activities.
The report said Iran was expanding its enrichment programme, defying a UN resolution of December 2006.
Iran believes the UN call for it to stop uranium enrichment is unacceptable.
It denies Western claims it is secretly trying to build nuclear arms, saying its nuclear programme is for purely peaceful, energy-producing purposes.
"Iran has obtained the technology to produce nuclear fuel and Iran's move is like a train... which has no brake and no reverse gear," President Ahmadinejad, quoted by ISNA news agency, told a gathering of clerics on Sunday.
Echoing this uncompromising tone, Deputy Foreign Minister Manouchehr Mohammadi said: "We have prepared ourselves for any situation, even for war."
However, Iran's chief nuclear negotiator Ali Larijani stressed Tehran was still ready for talks.
Iranians have protested outside Western embassies in Tehran
He urged UN Security Council members meeting in London not to continue their "hostile behaviour", and said Iran would act "proportionately" to any further pressure.
The BBC's Frances Harrison in the Iranian capital says that as the West piles on more pressure there is no sense that Tehran is ready to give in.
The rhetoric - at least - is as defiant as ever, says our correspondent.
Meanwhile the Tehran authorities have denied state media reports that a test rocket had been fired into space - a move that could also mean a huge advances in its ballistic missile programme. Tehran now says it was a sub-orbital research rocket.