ATHENS - A Greek detention centre for illegal migrants is an insult to human dignity and cholera could break out there at any time, the United Nations refugee agency (UNHCR) said on Tuesday.

Migrants have increasingly tried to cross into Greece by boat or through the land border with Turkey in recent years.

Following a Monday inspection visit to the island of Samos, off the western coast of Turkey, the agency said conditions at the centre "insulted human dignity" and violated human rights.

The crumbling tobacco warehouse was housing 391 mainly Asian refugees when it has a capacity for about 120, it said.

"Men, women and children sleep on the floors, the ceilings could cave in at any time, it is dirty, toilets are overflowing and cholera could break out at any time," UNHCR Public Information officer Kate Kehaioglou told Reuters.

"We have seen people with swollen arms and legs from insect bites and overall conditions are unacceptable," she said by telephone from Samos.

More than 3,500 illegal migrants have been arrested on Samos this year compared to a total of 1,580 in 2006, the U.N. agency said in a statement.

From 2002 to August 2007 more than 385,000 migrants attempted to enter Greece illegally, according to the Greek government. It has beefed up coast guard patrols, spending more on equipment and additional personnel.

But human rights groups say Greece has done little to improve camp conditions for refugees, pending decisions on their asylum status and potential deportation.

The government said it had completed construction of a new reception shelter on Samos.

"A new, modern facility to house the refugees has been constructed and is now ready," government spokesman Evangelos Antonaros said. "It will be open in the coming weeks to provide shelter for migrants."