In a written message on Thursday, Mojtaba Khamenei, who has yet to appear publicly since his nomination last Sunday, called for the vital Strait of Hormuz oil shipping lane to remain closed.

"The Iranians are showing defiance by choosing the son of Khamenei," a former diplomat told CNBC.

In his first statement since being appointed leader, Ayatollah Mojtaba Khamenei issues threat to US and says Tehran will 'take war reparations'