The United Nations nuclear watchdog said Thursday its top investigator Olli Heinonen, head of the agency's long-running investigations into Iran and Syria, would step down next month for personal reasons.

"We confirm that Heinonen informed the director general of his resignation for personal reasons to take effect at the end of August," said International Atomic Energy Agency spokeswoman Gill Tudor. "The DG has decided to respect his intention, with high appreciation for his long contribution to the agency.

"As for Heinonen's successor, nothing has yet been decided. But this post should be filled without delay through the normal practices."