TEHRAN, Iran — President Mahmoud Ahmadinejad vowed today that Iran won’t retreat “one iota” from its nuclear program, denying claims that it seeks atomic weapons.

Key ally Russia gave the Islamic Republic a major boost, rejecting tighter sanctions despite a U.N. watchdog report detailing suspected arms-related advances.

In his first reaction to the report, Ahmadinejad strongly chided the U.N. International Atomic Energy Agency — a day after it claimed Tehran was on the brink of developing a nuclear weapon — saying the IAEA is discrediting itself by siding with “absurd” U.S. accusations.

The comments, broadcast live on state TV, were a sharp rebuke to Western warnings that Iran appears to be engaged in a dangerous defiance of international demands to control the Islamic Republic’s nuclear ambitions.

In Paris, Foreign Minister Alain Juppe said France would support boosting sanctions against Tehran to an “unprecedented scale” if Iran stonewalls investigations, even as Israel and others say that military options are still possible.

But Russia, which has veto-wielding power on the U.N. Security Council, said new sanctions would be unacceptable.