Western states today begin the uphill task of persuading fellow members of the United Nations Security Council that a third round of modest economic sanctions would force Iran to come clean about its nuclear ambitions where earlier measures had failed.

At first sight, a package of measures revealed on Friday, and described by a senior European diplomat as "another turn of the screw", looked unlikely to break Tehran's intransigence in the face of demands that it reveal its nuclear secrets and suspend enrichment of uranium that could be used to construct a bomb.