Writing to Premier Giuseppe Conte and Foreign Minister Enzo Moavero Milanesi, Salvini slammed the "baselessness" of a recent letter from UN investigators which stated that his proposed decree violated human rights law enshrined in UN conventions.
The "irrelevant" remarks in the letter "were all the more reprehensible" ahead of the (May 22-26) European elections and given that the decree had not yet been approved by the Italian cabinet, Salvini wrote.
"It is unwarranted interference in our government's political programme, spawned by ideological preconceptions shared by those who oppose the new direction in our national migration policy," he added.
Salvini's letter said it was "unacceptable" to accuse Italy of violating migrants' human rights when it had "strongly and concretely" supported the work of the various UN agencies in Libya.
"The government must take a tough stance and hit back at the UN special rapporteurs," the missive underlined, demanding clarification on the value of Italy's budget contributions to the world body.
Salvini, a populist, had proposed the new security decree ahead of the European Parliament elections last week, where nationalist, anti-migrant parties were predicted - but failed - to make strong gains across the bloc.
Italy is obliged to rescue migrants in distress and cannot impede others from doing so according to the letter from six UN experts earlier this month. Nor can migrants be sent back to countries like war-torn Libya where they are in danger, it states.
Salvini's decree appears to be "yet another political attempt to criminalise search and rescue operations" that "further intensifies the climate of hostility and xenophobia against migrants," the letter stated.
The letter urged the Italian government to "halt" its approval of the decree, which would fine NGO-run ships that rescue migrants at sea and take them to Italian territory. Boats that save migrants also risk losing their licenses under the decree's provisions.
The UN experts, who urged officials to reply to their letter, asked for the withdrawal of two previous directives denying access to Italian ports to ships operated by NGOs that have rescued migrants off Libya's coasts.
It is not the first time Italy has come under fire from the UN since Salvini - who is also Deputy Premier - became Interior Minister in the populist government that took office last June.
In November, the Office of the High Commissioner for Human Rights had harshly criticised several aspects of an earlier security decree designed by Salvini that abolished "humanitarian protection" for migrants who could not be considered refugees under the current rules.
And last September, the UN's new High Commissioner for Human Rights Michelle Bachelet said teams of inspectors would be sent to Italy to "assess the reported sharp increase in acts of violence and racism against migrants, persons of African descent and Roma", sparking a furious reaction from Salvini.
( With inputs from IANS )