Senior envoys from Iran and the six countries trying to persuade it to compromise on its nuclear program have started talks - with a ranking US envoy attending for the first time. The six nations hope Iran will agree to stop expanding its uranium enrichment program. In exchange, they offer to hold off pushing for new UN Security Council sanctions against the Islamic Republic. If the two sides can agree, the next step would be formal negotiations which the six nations hope will result in Teheran fully scrapping its enrichment activities in exchange for a package of political and economic incentives. Enrichment can make either nuclear fuel or the material used in the fissile core of warheads.