NEW : Iran President tweets on `` 1 hour of constructive & pragmatic dialogue '' NEW : He criticizes U.S.-led strikes in Syria , yet opens door to `` a new atmosphere '' with U.S. NEW : Cameron : Despite `` severe disagreements , '' Iranians could help defeat ISIS . NEW : Obama says he will not allow Iran nuclear program to continue without his approval --
NEW : Iran President tweets on `` 1 hour of constructive & pragmatic dialogue '' NEW : He criticizes U.S.-led strikes in Syria , yet opens door to `` a new atmosphere '' with U.S. NEW : Cameron : Despite `` severe disagreements , '' Iranians could help defeat ISIS . NEW : Prime Minister : If they can help , `` we should welcome their engagement ''