Iran's nuclear chief says the country has increased the number of centrifuges enriching uranium to 6,000, the latest show of defiance to U.N. demands it halt the enrichment program.

In November, Iran said it had 5,000 centrifuges running at its enrichment plant in the central city of Natanz.

The nuclear chief, Vice President Gholam Reza Aghazadeh, says Iran will continue to install more centrifuges and enrich uranium to produce nuclear fuel for future Iranian power plants.

Uranium enriched to a low level is used as fuel in a reactor. Further enrichment makes it suitable for nuclear weapons.

The United States and its allies accuse Iran of pursuing nuclear arms. Tehran denies this, saying its program is aimed only at generating electricity.