It was last spotted on Monday by Malta's air force, which took pictures showing people apparently bailing water out of the overloaded vessel.

Maltese boats were later sent out but failed to find the immigrants - who are said to include children.

"We are very concerned for their safety," UNHCR spokeswoman Laura Boldrini told the BBC News Website.

She said those on board contacted relatives via a satellite phone on Monday.

They said the engine had broken down and water was coming into the boat.

"We are in a state of anguish," an Eritrean woman who lives in the Italian city of Bologna and whose brother appears to be on the boat told the UNHCR.

The UNHCR has urged governments in the region to increase efforts to locate and rescue them.

"According to our experience a missing boat can remain adrift for 20 days," she said.

"The authorities must continue the search so we can be sure that everything has been done," she said.

Ms Boldrini said she believed the boat could be located between Malta and the Libyan coast.

Malta - a small island that is now a EU member - is a leading destination for African migrants.

At least 3,500 reached the island over the past two years.