"It is essential because if the Iranian people cannot" produce their own nuclear fuel, "they will be dependent on outside sources and if these countries decide not to supply us, our stations will be useless," Khamenei said in a speech carried on state television.
The all-powerful leader, however, did not say when Iran would resume enriching uranium, an activity that is currently suspended in line with demands from the International Atomic Energy Agency (IAEA).
The IAEA is investigating Iran's bid to generate atomic energy, which the United States says is merely a cover for a weapons programme. Iran denies the charge.
The question of the nuclear fuel cycle is a serious concern at the IAEA, which fears it could eventually be used for military purposes.
Khamenei repeated the regime's assertions that this was not a risk.
"Iran is not trying to make an atomic bomb, because it does not need this to unravel its enemies," he said, employing a term commonly used to refer to the United States and Israel.
"It is by relying on faith, determination and unity that the peope can defeat their enemies," said Khamenei, who has the final word on all matters of state.
On Friday, the IAEA passed a British-French-German sponsored resolution that strongly criticised Iran for failing to cooperate enough with UN inspectors and calling for the probe to be stepped up and concluded within a few months.
The renewed pressure has sparked many officials here to call for a revision of ties with the IAEA, the international body charged with overseeing the nuclear Non-Proliferation Treaty (NPT).
"If the Europeans are in fact worried that we could have a nuclear weapon, we will reassure them: no, we do not want to have such weapons," Khamenei said.
"But if it bothers them that Iran masters nuclear technology and that this technology is home-grown, and if they want to oppose this, we say to them that the Iranian people will never accept the language of force," he added.
The US holds that Iran, OPEC's number-two exporter and world number two for natural gas reserves, does not need nuclear energy.
Khamenei replied to this, saying "our enemies want our oil to run out so that we will be dependent on them."
Russia is currently helping Iran build its first nuclear power plant at the southern city of Bushehr, but Moscow and Tehran have yet to agree on a fuel supply deal.
In the meantime, Iran has been pressing on with mastering the entire nuclear fuel cycle itself, a programme that is technically permitted by the