A top Iranian official Monday said nuclear scientist Mohsen Fakhrizadeh was killed in a new type of "complex operation", blaming arch-foe Israel and an exiled opposition group.

A poster depicting slain nuclear scientist Mohsen Fakhrizadeh is hung on a lamppost in the Iranian capital Tehran, on November 30, 2020. (Photo: AFP)

The "operation was very complex, using electronic equipment and no one was present at the scene," the secretary of Iran's Supreme National Security Council, Rear-Admiral Ali Shamkhani, told state TV.

The People's Mujahedeen of Iran (MEK) were "certainly" involved, along with "the Zionist regime and the Mossad", he added, referring to Israel's external security agency.