Italy says about 31,700 immigrants landed on Lampedusa last year
The UN's refugee agency says it is increasingly concerned for the welfare of nearly 2,000 migrants at a detention centre on a southern Italian island.
The centre on Lampedusa has a capacity of 850, forcing hundreds of people to sleep outdoors in the cold, the UN High Commissioner for Refugees (UNHCR) said.
It said Italy's new immigration policy was partly to blame for the conditions.
Italy reported a sharp rise in the number of migrants arriving by sea on the island from Africa in 2008.
The Italian interior ministry said about 31,700 immigrants had landed on Lampedusa last year - an increase of 75% from 2007.
Since Monday, nearly 400 would-be immigrants have arrived on the Italian island, the authorities say.
The centre was set up to temporarily house people rescued at sea while preparations were made to transfer them to various special centres in southern Italy to assess their needs.
However, since Italy changed its rules earlier this month, all rescued migrants have to remain in Lampedusa until decisions are made on their asylum bids.
In a statement, the UNHCR said the overcrowding was "creating a humanitarian situation of concern".
"We urge the Italian authorities to take all necessary steps to address the difficult humanitarian situation now unfolding in Lampedusa," said the head of the UNHCR's European division, Pirkko Kourula.
Correspondents say many would-be migrants are fleeing wars or poverty in countries such as Somalia and Eritrea and risk the dangerous Mediterranean crossing to enter Europe.
People smugglers charge them about $1,000 (£685; 710 euros) each to make the often stormy crossing in barely seaworthy vessels.