U.S. Secretary of State Condoleezza Rice said on Thursday Iran's resumption of atomic fuel research would signal its rejection of a diplomatic solution to the nuclear crisis, but stopped short of saying this would finally trigger a U.S. push for U.N. Security Council action.

Rice said she hoped "diplomacy has not been exhausted," but added that it was "becoming clearer" Iranians are not accepting a diplomatic compromise that constrains their nuclear ambitions.

Rice addressed State Department reporters after President Mahmoud Ahmadinejad said on Thursday Iran would resume atomic fuel research next week despite warnings from the West that this would endanger efforts to find a diplomatic compromise.

She said if Iran proceeded with sensitive nuclear research "it really will be a sign that they are not prepared ... to actually make diplomacy work."

Rice refused to be pinned down on a timeline for tougher diplomatic action, but said Washington will take the case to the Security Council at "a time of our choosing."