* Spain is fastest growing entry point to Europe

* UNHCR says country ill prepared to handle crisis

By Paul Day and Edward McAllister

MADRID/DAKAR, July 6 (Reuters) - The number of migrants crossing into Spain by sea from North Africa has doubled in 2017 from last year, outpacing the Libya-Italy route as the fastest growing entry point to Europe.

The United Nations Refugee Agency (UNHCR) says the spike in migrant boats is already putting a lot of stress on Spain's insufficient migration structures.

Escaping poverty and conflicts, more than 360,000 refugees and migrants arrived on European shores across the Mediterranean last year, according to the UNHCR. More than 85,000 have reached Italy so far this year.

Spain's interior ministry did not return calls and emails seeking comment.

While the Italian sea route remains the most popular overall with 59,000 migrants between January and May, up 32 percent from last year, the Spanish route further west has gathered steam with 6,800 migrants using it in the same period, a 75 percent increase from 2016.

In June, the trend was even more pronounced as 1,900 migrants, mostly young men originating from Guinea, Ivory Coast, Gambia and Cameroon, reached the shores of the Southern region of Andalusia, quadrupling the numbers registered the same month last year.

Further South, just as dramatic is the fall in the number of migrants spotted in the Agadez region of Niger, a key stop on the way to Libya from West Africa.

"People are talking about going to Spain. It seems like it is safer to go through Morocco to Spain than through Libya. The difference is that Libya doesn't have a president and Morocco does – there are not guns like in Libya," said Buba Fubareh, a 27-year-old mason from Banjul, Gambia, who tried and failed to get to Europe via Libya earlier this year.

Many African migrants passing through Libya have reported having been beaten up, detained in camps with no food or water and even traded as slaves before being held for ransom, forced labour or sexual exploitation.

UNPREPARED?

A similar reorganization has also taken place within the Western Mediterranean route itself, with the Alboran Sea, which connects North-Eastern Morocco and South-Eastern Spain, being now more popular than the previously favoured Gibraltar strait or Ceuta and Melilla land borders where policing has increased.

Migrant arrivals on the Spanish coastline averaged just under 5,000 a year between 2010 and 2016, according to government data, down from peak of 39,180 in 2006. It is on track to top 11,000 this year, government data shows.

The country was unprepared to handle vulnerabile groups such as victims of trafficking or unaccompanied minors and refugees who should be channelled through asylum procedures, the UNHCR said.

Spain has so far given a lukewarm response to a request from Italy to fellow European Union countries to allow rescue boats carrying African migrants across the Mediterranean to dock in their ports and help handle tens of thousands of arrivals.

"What is clear is that, they (Spain's government) have to get ready. They can't be caught unprepared. What started happening elsewhere in Europe in 2015 can't be allowed to happen here," spokeswoman for the UNHCR in Spain Maria Jesus Vega said.

"It's not yet an emergency, but you have to take into account that there are no structures here to deal with more arrivals." (Writing by Julien Toyer; editing by Ralph Boulton)