A final accord to curtail Tehran's nuclear ambitions could lead to a glut of Iranian oil hitting an already "oversupplied market," which would serve to pressure prices, oil expert John Kilduff said Tuesday, as negotiators in Vienna face a deadline that's expected to be extended.

Returning to the talks after consultations at home, Iran's chief diplomat insisted Tuesday he has a mandate to finalize a nuclear agreement, despite increased signs of backtracking.