Iran's chief nuclear negotiator, Ali Larijani, has said his country will suspend contacts with the UN's nuclear watchdog if sanctions are imposed.
Larijani's comments come ahead of a UN deadline on Iran's activities
He also said Iran would "hide" its nuclear programme if it was attacked.
US Secretary of State Condoleezza Rice said Iran's threats further isolated it from the international community.
The Security Council has set a deadline of 28 April for a freeze in uranium enrichment, the focus of concerns that Iran could acquire nuclear weapons.
The US is trying to rally support from the Security Council for tougher action against Iran, including sanctions - a move currently being resisted by Russia and China.
Speaking after a meeting with the Greek foreign minister during an official one-day visit, Ms Rice said Iran's threats were "emblematic of the kind of Iranian behaviour seen over the past couple of years".
Ms Rice said the international community was not prepared to allow Iran "under cover of a civil nuclear programme to acquire the technologies that could lead to a nuclear weapon".
Ms Rice said the Security Council must now issue something more concrete than last month's "presidential statement", which gave Iran 30 days to comply with International Atomic Energy Agency (IAEA) directives.
'Force not solution'
"Military action against Iran will not lead to the closure of the programme. If you take harsh measures, we will hide this programme. Then you cannot solve the nuclear issue," Mr Larijani warned.
"They [the Western countries on the IAEA board] have to understand they cannot resolve this issue through force," Mr Larijani told a conference on Iran's controversial nuclear energy programme in Tehran.
At the same conference, former President Hashemi Rafsanjani said Tehran had no intention of diverting nuclear material for a military programme at the moment.
The BBC's Frances Harrison in Tehran says the implication of his comments is that this might be possible in the future. Our correspondent adds that Mr Rafsanjani is still a key power broker in the Iranian administration.
Both men said they were keen on negotiations to reassure the West that Iran's programme is peaceful, but not negotiations to stop Iran having a nuclear programme altogether.
Iran insists its nuclear programme is for civilian energy purposes only. The US and several other nations say they do not believe this.
The IAEA says there is so far no proof that Iran is seeking nuclear weapons - but it talks of an "absence of confidence that Iran's nuclear programme is exclusively peaceful", and of a "policy of concealment" pursued by Tehran.