"These four people were not saboteurs. They cut the fences and entered the area to collect scrap iron and steel and sell it on the market," Mehr quoted Alavi as saying.
"In fact, they were thieves not nuclear saboteurs," said Alavi, adding they were "villagers who had done this before".
Alavi did not specify at which nuclear site the arrests were made.
Iran's nuclear chief Ali Akbar Salehi said earlier this month that four people suspected of attempting to sabotage one of Iran's nuclear plants were arrested.
In August last year, saboteurs blew up power lines supplying Iran's underground uranium enrichment plant at Fardo outside the central city of Qom.
In 2010, a US cyber-attack, reportedly carried out in collaboration with Israel, hit Iran's nuclear facilities. The Stuxnet virus was tailored specifically to target uranium enrichment facilities.
In recent years, Iran has detained a number of alleged US or Israeli agents accused of spying on, or attempting to sabotage, its nuclear programme.
