"Releasing fabricated news about Iran's arming of extremist groups in Afghanistan or other countries has become a boring repeated issue," ministry spokesman Ramin Mehmanparast told Iran's state-funded Press TV Tuesday.
The denial was in response to reports NATO had identified a cache of weapons seized in Afghanistan as coming from Iran and destined for the Taliban.
British Foreign Secretary William Hague said last week Britain was in "no doubt" about the source of the weapons.
"The detailed technical analysis, together with the circumstances of the seizure, leave us in no doubt that the weaponry recovered came from Iran," Hague said. "It is not the behavior of a responsible neighbor."
Mehmanparast said western countries were resorting to "fabricating news and leveling false accusations (against Iran) in order to divert public opinion's attention" after their military invasion of Afghanistan proved ineffective.