VIENNA, Austria -- Iran and six world powers are back at the negotiating table eager to come to terms on a comprehensive nuclear deal but deeply divided on what it should look like.

The two sides began meeting Tuesday in attempts to build on a first-step accord that temporarily curbs Tehran's nuclear activities in exchange for some sanctions relief. Implemented in January, the first-step deal is in effect for six months and can be renewed.

The six seek to minimize concerns that Tehran could use a peaceful nuclear program to make weapons. The Islamic Republic denies wanting -- or ever working on -- nuclear weapons.