Iran now has more than 5,000 centrifuges operating and enriching uranium at the country's central plant, the Iranian nuclear chief announced Wednesday, in Iran's latest defiance of U.N. demands that Tehran halt the controversial program.

The new figure is a significant jump from the 4,000 Iran said were up and running in August at the plant in the central Iranian city of Natanz.

The Iranian official, Vice President Gholam Reza Aghazadeh, said Iran will continue to install centrifuges and enrich uranium in order to produce nuclear fuel for the country's future nuclear power plants.

"At this point, more than 5,000 centrifuges are operating in Natanz and enriching uranium," said Aghazadeh, who is also the head f Atomic Energy Organization of Iran. He spoke to reporters during an exhibition of Iranian nuclear achievements at Tehran University.

Uranium enriched to low level is used to produce nuclear fuel. Further enrichment makes it suitable for use in nuclear weapons.

The United States and some of its allies accuse Iran of seeking to build nuclear weapons. Tehran denies the claim and insists it has the right under the Nuclear Nonproliferation Treaty to enrich uranium and produce reactor fuel.

The U.N. Security Council has already imposed three rounds of sanctions on Iran for its refusal to freeze the uranium enrichment program.

Flaunting Iran's defiance, Aghazadeh said the country won't suspend enrichment. "Suspension has not been defined in our lexicon," he added.

During the enrichment process, uranium gas is spun in a series of centrifuges known as "cascades" to purify it. Lower levels of enrichment produce reactor fuel — which Iran says is the sole purpose of the program — but higher grades can build a weapon.

At the exhibition, Iran for the first time put on public display one of its P-1 centrifuges and officials at the exhibition explained various parts of machine to visitors.

The P-1 centrifuge is the workhorse of Iran's enrichment program. It's run in cascades of 164 machines.

In February, Iranian officials confirmed that they have started using the IR-2 centrifuge, which can churn out enriched uranium at more than double the rate of P-1.

Iran has said it plans to move toward large-scale uranium enrichment that will ultimately involve 54,000 centrifuges.

In its latest report on Iran last month, the U.N. nuclear watchdog said that Tehran had not significantly expanded full or partial operation of nearly 4,000 centrifuges at its cavernous underground facility at Natanz, a city about 300 miles (500 kilometers) south of Tehran.

The International Atomic Energy Agency said the Islamic Republic was installing, or preparing to install, thousands more of the machines that spin uranium gas to enrich it — with the target of 9,000 centrifuges by next year.

IAEA officials could not be immediately reached for a comment on Aghazadeh's claim Wednesday.

Aghazadeh also said Iran has also made "good progress" in constructing a 40 Megawatt heavy-water reactor near Arak in central Iran. "The heavy water plant is experiencing a production beyond its capacity," he said without elaborating.

The West has repeatedly called on Iran to stop construction of the reactor, fearing it could be used as a second track toward building a warhead.

When it is finished, the Arak reactor could produce enough plutonium for a nuclear weapon each year, experts have said.

Aghazadeh also claimed Iran has conducted research on nuclear fusion — allegedly another one of its "priorities" — but didn't provide more details on the research. "It started long ago," he said.

Nuclear fusion is an energy producing process which naturally takes place in the sun and stars. Scientists have long sought a simple way to produce fusion in hopes of harnessing it as an energy source.

Also Wednesday, Iranian state television reported that the country successfully launched a second rocket into space, following up on the first such launch in February.

The rocket, entitled "Kavoshgar 2" or Explorer 2, made it to the lower reaches of space and returned to earth 40 minutes later on a parachute. It wasn't clear when the launch took place.

Iran has long held the goal of developing a space program, generating unease among world leaders already concerned about its nuclear and ballistic missile programs.