* Cyprus ammonium nitrate find at five tonnes * Suspect in custody denies its his * Ammonium nitrate can be powerful explosive NICOSIA, June 2 (Reuters) - Cyprus believes it may have foiled a major explosives attack, a security source said, in seizing nearly five tonnes of chemical fertiliser for a planned action Israel says bears the hallmarks of the Hezbollah guerrilla group. Authorities detained a Lebanese-Canadian in late May after finding ammonium nitrate, a potential explosive, in his basement. "With those kind of quantities something bad could have happened, and it was foiled," a security source told Reuters on condition of a**nymity.