"By the grace of God and with the good prayers of devotees, the gentleman (Ayatollah Khamenei) is in good health and is busy vigorously carrying out his plan according to his routine," the official, Mehdi Fazaeli, said on Twitter.
Fazaeli has worked in an office publishing Khamenei's work. His tweet was also reported by Iran's semi-official Fars news agency.
The statement about Khamenei's health appeared to be a response to reports by several news organizations, which referred to a tweet by a journalist in Arabic who said Khamenei had transferred duties to his son because of his health.
Khamenei has served as supreme leader since 1989, with the final say on all state matters. His health has been the subject of speculation for years.A security official in Israel, which closely follows events in its enemy Iran, told Reuters: "We are aware of these rumors, and regard them as rumors only.”