Internal differences are killing Opec and its ability to influence the markets has all but evaporated, top Russian oil executive Igor Sechin told Reuters in some of his harshest remarks ever about the oil cartel. Russia, which has been hit hard by the oil price collapse, was flirting with the idea of co-operating with Opec in recent months until tensions between Opec members Saudi Arabia and Iran ruined a global deal to freeze output. Sechin - one of the closest allies of President Vladimir Putin - was the only Russian official to consistently oppose the deal with Opec even after the Kremlin effectively endorsed the plan.