"What is happening in front of our eyes in Italy is an unfolding tragedy," Filippo Grandi said.
"This cannot be an Italian problem alone," he added.
Italy has threatened to stop vessels of other countries from bringing migrants to its ports, as it warned the EU the situation was unsustainable.
Italy needs more support as it deals with large numbers of migrants crossing the Mediterranean from Africa, the UN's refugee agency has said.
"What is happening in front of our eyes in Italy is an unfolding tragedy,"Filippo Grandi said.
"This cannot be an Italian problem alone," he added.
Italy has threatened to stop vessels of other countries from bringing migrants to its ports, as it warned the EU the situation was unsustainable.
Mr Grandi, who is the United Nations High Commissioner for Refugees, said 12,600 migrants and refugees had arrived in Italy last weekend alone.
In total, he said, 83,650 people have reached Italy by sea since the beginning of the year - a 20% increase on the same period in 2016.
An estimated 2,030 have lost their lives in the Mediterranean this year.
The vast majority of migrants making their way to Italy across the Mediterranean set off from Libya.
Libya is a gateway to Europe for migrants from across sub-Saharan Africa and also from the Arabian peninsula, Egypt, Syria and Bangladesh. Many are fleeing war, poverty or persecution.
The UNHCR said that among the arrivals in Italy there was an alarmingly high rate of unaccompanied children or victims of sexual or gender-based violence.
UN calls for more support for Italy
02 July 2017
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Italy needs more support as it deals with large numbers of migrants crossing the Mediterranean from Africa, the UN's refugee agency has said, BBC reports.
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