"When a rootless child-killing criminal like @Netanyahu calls himself 'the frontline of American civilization', you can’t blame Americans if they feel like suicide," Ali Larijani wrote on X.
US Secretary of State Marco Rubio met Netanyahu on Monday and both hailed the unity of their two nations in confronting threats they say Iran poses.
Larijani is a relative moderate and veteran insider who was elevated to lead Iran's Supreme National Security Council following a 12-day war in which Israel and the United States pounded Iranian nuclear and military sites, leaving hundreds dead.
It was unclear what Larijani meant by the assertions. "Rootless cosmopolitan" was a slur deployed in the Soviet Union directed at intellectual figures and particularly Jews, and the descriptor has been consistently deployed by anti-Semitic polemicists.
Critics of Israel have frequently scorned the relatively recent arrival of its leaders or their families from other countries in the twentieth century.
The Israeli premier's father, who was born in Poland as Benzion Mileikowsky, later adopted the Biblical surname Netanyahu and became a preeminent scholar or early modern Spain.
Larijani's assertion on the United States was less direct and could have been a reference to a fraught moment in American politics as political polarization deepens in the wake of the assassination of prominent political commentator Charlie Kirk.
Holding a PhD in philosophy and the rank of brigadier general in Iran's Islamic Revolutionary Guard Corps, Larijani has long plied a deft course around public debates which have divided Iran's ruling factions.
Larijani also serves as Supreme Leader Ali Khamenei's personal representative to the key decision-making SNSC, an apparent key endorsement by Tehran's top authority.
His new mandate is widely seen as righting Iran's security tack after lapses allowing Israeli attacks to kill nuclear scientists along with hundreds of military personnel and civilians.
In the sensitive new role, Larijani has stepped up the frequency and edge of his social media criticism of Israel and the United States, especially following the 12-day war in June.