U.S. President George W. Bush said in Washington on Friday (October 27) he was aware of "speculation" that Iran has started enriching uranium in a second network of centrifuges. "Whether they doubled it or not, the idea of Iran having a nuclear weapon is unacceptable," Bush told reporters during a meeting with North Atlantic Treaty Organisation (NATO) Secretary General Jaap de Hoop Scheffer. Iran's student nuews agency ISNA reported earlier in the day that Iran has started enriching uranium in a second network of centrifuges, expanding a programme which the West fears is intended to make nuclear bombs. ISNA quoted an "informed source" as saying "the injection of gas was carried out" in the past week. Uranium UF-6 gas is injected into cylindrical centrifuges which spin at supersonic speeds to produce enriched material. The process can make fuel for power plants or material for atomic bombs. Iran, the world's fourth largest oil exporter, says its atomic programme aims to meet energy needs. But it has failed to convince world powers, who are threatening U.N. sanctions after Tehran failed to heed a U.N. demand to halt enrichment work. Diplomats said this week Iran had started "dry testing" a network of 164 centrifuges, known as cascades, to go with an original network that yielded Iran's first batch of enriched uranium suitable for power plant fuel. The first cascade of centrifuges produced a tiny amount of low-enriched uranium in April. Western intelligence experts estimate Iran remains three to 10 years away from an industrial-scale operation of thousands of centrifuges that could yield enough fuel for nuclear bombs.