Iran's nuclear chief says the country has increased the number of centrifuges enriching uranium to 6,000, marking Teheran's latest defiance of UN demands to halt its nuclear program. Vice President Gholam Reza Aghazadeh said Iran will continue to install more centrifuges and enrich uranium to produce nuclear fuel for future Iranian power plants. In November, Iran said it had 5,000 centrifuges running at the plant in the central city of Natanz. Uranium enriched to a low level is used to make nuclear fuel. Further enrichment makes it suitable for nuclear weapons. The United States and its allies accuse Iran of pursuing nuclear arms. Teheran denies this, claiming its nuclear program is geared only toward generating electricity.