The UN has asked for access to migrants landing illegally in Italy as Rome presses ahead with the mass expulsions by air that began at the weekend.
Italy is seen as the gateway to Europe by migrants
The UN's refugee agency said it wanted to ensure genuine refugees were not expelled along with illegal migrants arriving by sea from North Africa.
Italy has sent at least 11 planeloads of migrants to Libya since Friday, in a dramatic change of policy.
It acted after more than 600 arrived on the island of Lampedusa in one night.
Up to 600 people are known to have been deported and the tough policy has caused mounting concern among humanitarian agencies.
The plight of desperate illegal migrants who are often put to sea in old boats by human traffickers was highlighted by news that a ship had sunk off Tunisia on Sunday, with the confirmed loss of 22 lives.
On Monday, an Italian warship intercepted a wooden boat crammed with some 150 people in international waters off Lampedusa and summoned the Tunisian navy to escort it back to the North African coast.
Others were spotted heading from Libya to Lampedusa in a rubber dinghy, according to an Italian coastguard quoted by the Associated Press news agency.
Demand for access
Raymond Hall, director of the UN refugee agency's (UNHCR) Regional Bureau for Europe, said all asylum-seekers "should have access to a fair procedure".
"UNHCR should have access to them and we stand ready to assist Italian authorities in ensuring that those who do need protection get it," he said.
He added that the agency appreciated the "very strong pressures" on Italy.
His comments joined a chorus of criticism of the Italian policy.
Stefano Savi, director of Doctors Without Borders in Italy, said Italy had a humanitarian duty to potential refugees.
"Many of the men and women who head to Italy are making dangerous trips to flee war and persecution," he said, in a statement issued jointly with Amnesty International.
The African Union condemned the expulsions, complaining that immigrants' human rights were "less and less observed by authorities of host countries".
Libya is not a signatory to the UN's Geneva Convention on refugees and has been accused of abusing the rights of migrants.
However, Italian officials quoted by AFP news agency said they were allowing arrivals from Eritrea, Somalia and Ethiopia access to the asylum process, while those from other countries were being expelled.
'Assault on Italy'
Italian Interior Minister Giuseppe Pisanu spoke of an "assault on the Italian coast... by criminal gangs who ruthlessly cash in on illegal immigration".
In an abrupt change of policy, the Italian government decided at the weekend to start sending migrants from Africa, Asia and the Middle East back to Libya by plane.
Italy's interior ministry undersecretary Alfredo Mantovano told the La Stampa newspaper on Monday that sending migrants back was a method the country would use "increasingly".
"If they know they will be sent back, maybe they won't set off?" he said in La Stampa newspaper.
At least three flights reportedly took off on Monday.
The island is the nearest geographical point of arrival in the European Union from north Africa.
Back to Libya
The BBC's Rana Jawad reports from Tripoli that the migrants will be held in detention centres before being flown back to their country of origin.
The Tunisian navy rescued 11 people from the boat which sank on Sunday - only an hour after setting off for Italy carrying would-be migrants from Morocco and Tunisia.
"Half an hour after they left, the boat split under the weight of the passengers and broke in two, throwing everybody into the sea," said one witness.
As of Monday, 42 people were still missing, believed drowned.
Navies from European Union countries are due to start patrolling international waters around Lampedusa this week, in an attempt to deter people trafficking.