A huge recent surge in migrants arriving in Italy by sea is set to continue, the International Organization for Migration has warned.
Nearly 6,000 have arrived since Tuesday alone, it says.
In the week to 13 April, arrivals in Italy were 173% higher than the previous week, while arrivals in Greece were 76% lower.
Officials in Libya say they fear the closure of the migrant route through Greece is leading to the surge.
A deal between Turkey and the EU came into force last month with the aim of deterring migrants, mainly Syrians and Iraqis, from making the crossing between Turkey and Greece.
However, the IOM said there was no evidence yet that the Turkey-EU deal was linked to the latest surge. Those currently arriving in Italy from Libya are predominately citizens of African countries.
IOM spokesman Joel Millman said that of the 6,021 migrants and refugees making perilous sea crossings since Tuesday, only 174 had reached Greece.
He warned that the return of better weather would probably mean persistently high numbers making the crossing to Italy.
Rescued migrants who reached Italy said they had all travelled from Libya, mostly in overcrowded dinghies.
Federico Soda, the head of the IOM's office in Rome, said in the statement: "Many of them were from sub-Saharan Africa, and we have noticed an increase in numbers from the Horn of Africa, particularly Eritreans.
"There have been very few Syrians leaving from Libya in recent months."
Italy has asked its local authorities to find another 15,000 beds for asylum seekers.
The BBC's Orla Guerin, who has travelled to Libya, says one senior figure there said that hundreds of thousands could depart for Italy.
She says Libya's new unity government is struggling to get control of its capital, let alone the long coastline. With three rival administrations and numerous competing militias, the resulting chaos and insecurity has made smuggling a growth industry.
Earlier this week, the president of the European Council also warned Italy and Malta to expect large numbers of would-be migrants departing from Libya.
Donald Tusk told MEPs that it would not be possible to apply the same approach used for the Balkans to Libya.
So far this year, the IOM has recorded more than 23,000 arrivals in Italy and more than 153,000 in Greece.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.