Story highlights The migrants were picked up from more than 40 overcrowded boats in the Mediterranean

About 18,000 people have arrived by sea in Italy so far this year, the UNHCR says

Last year, nearly 43,000 migrants reached Italy, more than a quarter of them from Syria

The Italian navy has rescued about 6,000 people in the past four days from more than 40 overcrowded boats in the Mediterranean, the United Nations' refugee agency said Friday.

Its figures give a new indication of the scale of the challenge faced by Italian authorities in keeping up with the flow of migrants seeking to reach European soil.

About 18,000 people have arrived by sea in Italy so far this year, while in 2013, the number of arrivals was near 43,000, the UNHCR said. Of those who arrived last year, the largest group, more than 11,300, were Syrians fleeing the conflict in their homeland.

Of those picked up off the Italian islands of Sicily and Calabria in the past four days, a large number were women and children, including newborns and unaccompanied minors, UNHCR spokeswoman Melissa Fleming said in a prepared statement Friday.

Many were fleeing violence, conflict and persecution, the statement said. The migrants, who had set off from Zwara, in Libya, in search of safety within the European Union, have now been taken to Italian ports.

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The UNHCR has urged countries to work together to rescue people at sea at the same time as looking for legal migration channels that will prevent people from risking their lives at sea.

"The Mediterranean is one of the busiest seaways in the world, as well as a dangerous sea frontier for many asylum-seekers trying to find safety in Europe," the statement said.

"The challenges of protecting refugees traveling irregularly by sea in search of safety, often together with people moving for other reasons, are complex."

The agency also says that the reception centers in which asylum seekers are held after they are rescued must be adequate.

Last October, the world was shocked by the deaths of hundreds of people in shipwrecks off Italy's coasts. More than 300 died in one shipwreck alone off the tiny island of Lampedusa.

The Italian government set up a rescue operation after that tragedy, the UNHCR said, which has saved more than 20,000 people at sea.