Enriching uranium creates fuel for nuclear power plants but can also, if taken to higher levels, produce the material for weapons and Iran's growing capacity in this process is at the center of its dispute with the international community.

The UN Security Council has already passed four sets of sanctions against Iran to try and force it to stop enriching uranium.

“The new enrichment facilities will be built inside mountains,” said Ali Akbar Salehi, head of the Atomic Energy Organization. Revelations a year ago of a previously undisclosed enrichment facility in a secret mountain base near the city of Qom inflamed international suspicions over Iran's enrichment program.

British Prime Minister David Cameron's spokesman Steve Field said that Salehi's announcement was a cause for concern. “The reports that we have seen this morning certainly do not give us any comfort that Iran is moving in the right direction,” Field told reporters.

Iran has an industrial-scale, internationally supervised enrichment site in Natanz, central Iran.