Last year, 75 per cent of those who arrived in Italy, mostly from sub-Saharan Africa and the Horn of Africa, sought political or humanitarian asylum, and half of those obtained it, according to UNHCR figures.
Speaking on Friday, Ronchi said that the UN's objections showed they were "not aware of the situation in Italy".
"I would advise it to be better informed to avoid being demagogic, because some statements can be offensive," he said.
Ronchi said that the tens of thousands of people who had been "saved, protected and welcomed by Italy" were an "example to the whole world".
The Medecins Sans Frontieres (MSF aid agency described Italy's actions as a "terrible event".
"Far from being a historic event as the Italian government suggests, this forcible and cynical return is contrary to international laws," Loris de Filippi, the head of MSF-Italy, said.
L'Osservatore Romano, the Vatican's newspaper, said it was "worrisome" that the migrants were not able to request political asylum.
Sending back the migrants to Libya "violated international rules on refugee rights," Monsignor Agostino Marchetto, the Vatican's top official for migrant issues, said.
'Inhuman and degrading'
Human Rights Watch said Libya had a terrible record in dealing with migrants, who it said were sometimes subjected to indefinite detention "often in inhuman and degrading conditions".
But Roberto Maroni, Italy's interior minister, said Libya had adequate facilities for interviewing migrants to determine if they needed asylum.
|Napolitano warned of a "worrisome" increase in intolerance toward immigrants in Italy [AFP]
He also praised Libya for taking in the migrants as called for by a new Italian-Libyan border patrol agreement, describing the events as a "historic day".
The European Commission said on Friday that it needed more details before it could decide whether Italy's decision to send the immigrants back to Libya respected European Union laws.
"We do want to know what is going on. At this stage we don't have any details but we are going to monitor the situation," a justice affairs spokesman for the commission said.
Giorgio Napolitano, Italy's president, also appeared to join the debate on Friday when he warned of a "worrisome" increase in intolerance toward immigrants in Italy.
The president, whose role is largely symbolic, said increases in migrant smuggling "risked creating a diffuse perception of insecurity and worrisome instances of intolerance".
The government is pushing fresh legislation through on the issue as it looks to introduce a new offence of "clandestine immigration" to the law books.
If the plan becomes law, it would become a crime to be an illegal immigrant.
Italy's left-wing opposition on has likened the proposed new law to the racial purity laws introduced in Italy in the 1920s by the fascist regime of Benito Mussolini.
An Italian deputy from the Northern League party sparked controversy on Friday after he said public transport seats in his constituency of Milan should be reserved for Italians only.
Matteo Salvini, a Milan town councillor and vice-secretary for the Northern League, said: "Reserve seats for people from Milan, like those reserved for handicapped people and women. Because soon, if immigration does not stop, they [local Italians] will become a minority to protect."
As condemnation from the left and right parties spread, the head of the Northern League's local branch said the proposition by Salvini, who is a candidate for the June European elections, was "a joke, a provocation".