Milei, a maverick right-wing libertarian, in the past called Francis an "imbecile," a "son-of-a-bitch preaching communism" and the devil's man on earth.
In his letter, the pope also called, once again, for the release of those hostages still being held by Hamas.
Blessings were allowed last month in a document called Fiducia Supplicans (Supplicating Trust), which has caused widespread debate in the Catholic Church.
Netanyahu sent a letter to the Pope on Sunday after appealing to 33 female leaders worldwide, including First Lady Jill Biden, in previous weeks.
Defending Israel's right to self-defense, Chief Rabbi of South Africa Dr. Warren Goldstein accused the Pope of "primitive pacifism."
Pope Francis has called several times for an immediate ceasefire in Gaza and for the release of all hostages taken by Hamas.
Pope Francis, today, also expressed concerns over the morality of Israel’s actions in the Gaza Strip.
An Israeli Foreign Ministry spokesperson said the incident was still under review and had no immediate comment on the pope's words.
In previous guidance from 2016, the Vatican said the ashes must be kept in "sacred places", and therefore neither at home, divided among family members nor scattered to the wind.
The attack was carried out in a university gymnasium in Marawi, a city in the south of the country besieged by Islamist militants for five months in 2017.