Iran said Sunday its nuclear program is irreversible, issuing yet another rejection of a UN Security Council deadline to cease enriching uranium that expires in five days.

Earlier this month, Tehran announced for the first time that it had enriched uranium using 164 centrifuges, a step toward large-scale production of nuclear fuel that can be used either in atomic weapons or in nuclear reactors for civilian electricity generation.

"Nuclear research will continue. Suspension of (nuclear activities including uranium enrichment) is not on our agenda. This issue is irreversible," Foreign Ministry spokesman Hamid Reza Asefi told reporters.

The United States and some allies charge Iran is using the program as a cover for weapons production. Iran says it is designed only for power generation.

The Security Council deadline of Friday is not binding, but the United States and Britain said Iran must comply or the two countries would seek a resolution to make the demand compulsory, which would raise the possibility of sanctions.

"Iran won't give up its rights and has prepared plans for any eventuality," Asefi said.

The spokesman said a Russian compromise plan for joint uranium enrichment was still on the table.

Ali Asghar Soltanieh, Iran's envoy to the International Atomic Energy Agency, the UN nuclear watchdog, Saturday spoke of a "basic agreement" between Iran and Russia to set up a joint uranium enrichment firm on Russian soil.

The announcement was a repeat of a similar declaration by Iran and Russia in February but details have never been worked out.

"Necessary grounds need to be prepared for its implementation," Asefi said. It still remains unclear whether Iran would entirely give up enrichment at home, a top demand of the West, or if the joint venture would be complementary to the existing enrichment inside Iran.

Asefi insisted Sunday that Iran has not used any advanced P-2 centrifuges in its enrichment of uranium.

Such a device would be a vast improvement over the current P-1 centrifuges, which Iran has said it used to enrich uranium.

Iran's hard-line President Mahmoud Ahmadinejad claimed last week that his country was conducting laboratory research on the advanced P-2 centrifuge, which could be used to more speedily create fuel for power plants or atomic weapons.

"We have not so far used P-2 centrifuges. What we have used has been P-1," Asefi told reporters.

The spokesman, however, said Iran had the right to work on P-2 centrifuge.

"No one can deny us of such a work," he said.

Iran has vowed it would never give up its right under the Nuclear Nonproliferation Treaty to enrich uranium and produce nuclear fuel.