Diplomats say Iran has started a new round of uranium conversion, despite international calls for it to halt sensitive nuclear work.
Officials close to the International Atomic Energy Agency confirmed Wednesday that conversion had resumed - an initial step in preparing nuclear fuel.
Iran conducted an earlier round of nuclear work in August, prompting the European Union to break off talks with the Islamic Republic about its nuclear program.
The United States has accused Iran of secretly trying to develop nuclear weapons, and has called for the country to be referred to the U.N. Security Council for possible sanctions.
The U.N. atomic agency is to meet next week to consider the request.
Iran says its nuclear program is peaceful and intended to generate electricity.
Some information for this report provided by AFP and Reuters.