The head of the U.N. nuclear agency says Iran continues to build centrifuges to enrich uranium in defiance of sanctions, but he emphasized that Tehran's progress has been slow.

IAEA head Mohamed ElBaradei

Speaking on the sidelines of a nuclear power conference in Berlin Thursday, Mohamed ElBaradei said Iran has about 3,300 centrifuges, up from about 3,000 last year.

The head of the International Atomic Energy Agency said that rate was "not very fast," and he urged Tehran not to speed up the process.

Last week, Iranian President Mahmoud Ahmadinejad announced Iran planned to install 6,000 new centrifuges to enhance the country's ability to enrich uranium.

Highly enriched uranium can be used in nuclear weapons, but Iran says its nuclear program is to generate electricity.

The IAEA has been investigating the possibility that Iran's enrichment program might be aimed at developing nuclear weapons.

The United States and other Western countries have accused Iran of trying to develop such weapons. 

Iran currently faces three sets of United Nations Security Council sanctions and international isolation for its refusal to halt uranium enrichment.

Some information for this report was provided by AFP, AP and Reuters.