(Refiles to clarify IOM, UNHCR role in paragraph 7; no other changes to text)
* Graphic on Mediterranean arrivals: http://tmsnrt.rs/2r3fFjQ
By Gabriela Baczynska
BRUSSELS, July 3 (Reuters) - Planned new centres around the Mediterranean to handle migrants will be no silver bullet solution to the European Union's immigration challenge, says a U.N. agency of the idea it will be asked to implement.
Irregular migration across the sea has been dramatically reduced, and only about 45,000 people have made it to Europe that way this year. But the hot-button issue is driving the EU's political agenda.
Last week, EU states agreed to tighten their external borders and spend more in the Middle East and North Africa to bring down the number of arrivals.
Chancellor Angela Merkel, trying to save her coalition, on Monday agreed to set up migrant camps on the German border, highlighting how the EU is unable to agree on joint migration policies and governments increasingly go it alone.
One thing EU leaders have agreed is to look at setting up "disembarkation platforms" to handle those rescued from the dangerous crossing. Most are brought ashore in Italy, but more than 1,300 people perished this year.
"The Mediterranean is a shared space, north-south. We have a joint responsibility to govern what happens in that space, including avoiding that people drown," Eugenio Ambrosi, the head of the International Organization for Migration's (IOM) EU mission told Reuters.
The IOM and its sister U.N. agency for refugees, the UNHCR, would assist in running the new sites.
Ambrosi said 10 existing migrant centres in Greece and Italy could first be beefed up and new ones could then be added in Malta. But opening others on the southern rim of the Mediterranean - as some EU states want - would take time.
"Before going outside of Europe, asking other countries to help, we have to make sure that enough European countries help each other," Ambrosi said in an interview.
Eventually, depending on where in the Mediterranean they were rescued, people would be taken to EU or African centres.
The much-publicised idea of Mediterranean camps would only work together with opening up more legal ways to get to Europe from non-EU countries, Ambrosi said.
EU states would have to share out legitimate asylum seekers from the centres, an idea that has divided them bitterly since 2015.
As more than a million people entered the EU in 2015, overwhelming Italy, Greece and Germany, eastern nations led by Poland and Hungary refused to help by taking in a share.
With this internal dispute still festering, the EU will turn to Tunisia and Morocco to host new sites. The African countries have a good opportunity to bargain hard.
Ambrosi said he opposed locating migrant centres in strifetorn Libya and said populists in the EU failed to recognise how far the number of arrivals had dropped since 2015.
"It's not a migration issue, it's a political and functioning-of-the-EU issue," he said. "There is no quick fix, there has never been." (Reporting by Gabriela Baczynska; editing by Andrew Roche)
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