Ayatollah Ali Khamenei was speaking two days after the United Nations atomic agency released a report that for the first time said Iran was conducting experiments whose sole purpose was the development of nuclear arms.
"The enemies, particularly the United States and its pawns and the Zionist regime, should know that the Iranian nation does not seek to invade any country or nation," he said, addressing officers at a military academy in Tehran.
"But Iran will strongly respond to any invasion or attack with such power and in a way that the aggressors and invaders will be smashed from the inside. Anybody who takes up the idea of an attack on Iran, should get ready to receive a strong slap and an iron fist."
His defiant rhetoric indicated that Tehran, which insists it is pursuing nuclear energy for peaceful purposes, is set on a confrontation with the West.
The supreme leader, who has final say on all state matters in Iran, rations his interventions in international affairs, and appeared to want to send a message that the Iranian establishment was united in its commitment to hit back against any pre-emptive attack.Diplomatic sources said however that less confrontational elements in Iran's power structures will have been unnerved by the detailed report assembled by the International Atomic Energy Agency, and by recent bellicose rhetoric from Israel.
William Hague, the Foreign Secretary, has described the world as entering a "dangerous new phase" regarding Iran.
The European Union has begun discussing new sanctions against Iran, following the lead of Britain and France, whose foreign minister called for a meeting of the UN Security Council on Iran. Russia and China meanwhile yesterday both agreed that Iran should not be subjected to new sanctions, highlighting the divisions in the international community.
Western powers will also call for Iran to be reported to the Security Council by the International Atomic Energy Agency, which meets next week in Vienna.
The administration of US President Barack Obama is meanwhile coming under mounting pressure from hawkish senators in Washington to devise much tougher sanctions than any of the UN or unilateral measures imposed over the past six years.
They want to see sanctions on the Central Bank of Iran that would effectively cripple the Iranian economy and probably send oil prices soaring.
Senator Chuck Schumer, a New York Democrat, will soon introduce a bill requiring an investigation by the administration into the bank's role in Iran's illicit nuclear activities. If positive indications are found, sanctions would be automatically triggered.
The bill is likely to be passed, given that 92 out of 100 senators signed a letter in drafted by Sen Schumer in August calling for the effective collapse of the CBI.
Sen Lindsey Graham, a Republican from South Carolina, told the Foreign Policy website: "We've got to make a decision: What's the biggest threat to the world, a nuclear-armed Iran or sanctions that would hurt us and the people of Iran? You've got two choices, the policy of containment or the policy of pre-emption. I'm in the pre-emption camp. I don't think containment works."
The Obama administration has so far downplayed expectations of pushing through a fifth round of UN sanctions while it considers it options.
Diplomatic sources said that Washington had also been taken aback by the failure of Benjamin Netanyahu, the Israeli prime minister, to offer assurances that he would not bomb Iran unilaterally.
By Alex Spillius taken from http://www.telegraph.co.uk/news/worldnews/middleeast/iran/8882093/Irans-supreme-leader-warns-West-of-strong-slap-and-iron-fist.html