Article III: Each non-NWS party undertakes to conclude an agreement with the IAEA for the application of its safeguards to all nuclear material in all of the state's peaceful nuclear activities and to prevent diversion of such material to nuclear weapons or other nuclear explosive devices.
http://en.wikipedia.org/wiki/Nuclear_Non-Proliferation_TreatyThe UN has imposed sanctions for exactly that reason, they cannot verify that Iran does not have a weapons programs because Iran is not cooperating with IAEA inspectors and has violeted safeguard seals on the enrichment process:
http://news.bbc.co.uk/2/hi/middle_east/6205295.stmDICKEY: Do you have any indication that there is some other completely separate Iranian nuclear-weapons program?
ELBARADEI: No, we don´t. But I won´t exclude that possibility.
DICKEY: But there´s another problem. Even if the declared nuclear research is all that Iran has going, there´s nothing in the Non-Proliferation Treaty itself to prevent them from enriching uranium - which they say is their right. They could get to the point of producing their own nuclear fuel, or bomb material, then tell you, "We´re pulling out of the treaty."
ELBARADEI: Sure. And if they have the nuclear material and they have a parallel weaponization program along the way, they are really not very far - a few months - from a weapon. We need to revisit the treaty, because that margin of security is unacceptable. But specifically on Iran, the board is saying, "You have a right under the treaty to enrich uranium, but because of the lack of confidence in your program and because the IAEA has not yet given you a clean bill of health, you should not exercise that right. In a way, you have to go through a probation period, to build confidence again, before you can exercise your full rights."
http://www.iaea.org/NewsCenter/Transcripts/2006/newsweek12012006.html