* Smuggling networks from sub-Saharan Africa deeply embedded
* NAfrica-Italy route "dramatically more dangerous"
* Efforts to counter trafficking undercut by conflict
* Smugglers adapt by switching departure points, boats
* "Only assistance ... is promises," coastguard says
By Aidan Lewis and Ahmed Elumami
TRIPOLI, June 2 (Reuters) - After a flurry of boat departures that sent hundreds of migrants to their deaths in the Mediterranean, survivors told police they had been kept for weeks on one meal a day in holding houses near the Libyan shore.
Then they boarded the rubber or wooden vessels, but only those co-opted to run or drive the boats were given life-jackets, according to accounts given to Italian police.
As calmer summer weather begins, European officials who struck a deal with Turkey to block crossings to Greece have been scrambling for ways to shut down flows on the other major sea route into the EU from Libya.
They hope a U.N.-sponsored government that arrived in Tripoli in March will bring stability, and the EU last week agreed to help to train Libya's coastguard.
But Libyan officials describe themselves as under-resourced and helpless against powerful smugglers who go about their trade with impunity, adapting swiftly to new conditions.
At least 880 migrants and refugees died trying to cross the Mediterranean last week, the U.N. refugee agency said. The route between North Africa and Italy was "dramatically more dangerous" than the one to Greece, it said, with the chances of perishing estimated at one in 23.
The new government faces a complex challenge asserting its authority, while efforts to counter people trafficking were thrown into disarray by the conflict that followed Libya's 2011 uprising, and the coastguard feels abandoned.
"The only assistance we have been offered so far is promises," said Colonel Abdulassmad Massoud of the coastguard in Tripoli.
Italy says arrivals so far in 2016 are down two percent on last year at about 40,000 landings, mostly from countries like Nigeria, Eritrea, Gambia and Somalia. There is little sign of a return of Syrian migrants, tens of thousands of whom were travelling through Libya until they switched to the Turkey-Greece crossing in 2015.
Instead, smugglers enriched by those Middle Eastern migrants are again working long-established routes across thousands of kilometres of hostile desert between sub-Saharan Africa and western Libya.
As migrants pass through Libya, they enter a system marked by abuse, corruption and a near-complete vacuum of state authority.
Many African migrants remain in the system for months or years, some settling or returning home and some raising money for an onward journey to Europe. A minority turn to crime or end up fighting in Libya's fitful conflict.
"All the smugglers are connected and they pass the migrants on between each other," said Ibrahim Shawish, the mayor of Murzuq, a desert town about 350 km from Libya's southern border.
Traffickers can cross that border freely he said, and convoys of as many as 25 vehicles then head north unhindered. "Sometimes there's a checkpoint at the entrance of a city, but they can always take a different route."
There is no policing and little external aid. Germany has pledged 4.5 million euros ($5 million) to support communities in Sabha and Gatroun, two key southern areas on migrant routes, but Libyan officials complain that international efforts are too focussed on blocking crossings at sea.
Though the EU has warned of hundreds of thousands of displaced people who could cross the Mediterranean, officials and aid workers in Libya say a lack of data makes an accurate estimate impossible.
The International Organization for Migration has identified 235,000 migrants in Libya, but says the real number is likely to be between 700,000 and one million.
Migrants who are arrested often end up back in the smuggling networks after being released or even deported, officials and aid workers say.
"It makes no difference because the flow coming in is much greater than the number being deported, and the ones who are sent back to their countries are returning because there is no security on the borders," said Salem Ashwin, a migration official at Tripoli's international cooperation ministry.
Lawless Libya provides fertile ground for the smugglers, who often work with militias that hold real power on the ground.
In an unusual backlash, residents in Zuwara, a long-time stronghold for people smuggling about 50 km from the Tunisian border, demanded action against the traffickers after bodies washed up on the beaches last year. Some smugglers were arrested and some fled, according to Zuwara Mayor Hafed Bensassi.
But other departure points have opened up, with at least half a dozen between Zuwara and the port city of Misrata, about 300 km to the east. Most of the recent departures appear to have been from around Sabratha, which was in turmoil earlier this year as local brigades fought Islamic State militants.
"Until we have enough technology to watch the whole coast they'll always find gaps," said Salem, the ministry official.
Smugglers offer different prices for different types of boats, which range from seaworthy fishing vessels with radar systems to cheap inflatable dinghies with improvised wooden bases.
Even in the Mediterranean, they appear able to skirt round restrictions with relative ease.
People trafficking is intertwined with that of drugs and fuel, coastguards say, and new supplies of migrant boats are brought in on smuggling vessels that arrive from Malta and Egypt.
"Most of the fuel smuggling boats are carrying in migrant smuggling materials in front of the eyes of the Europeans," said Colonel Ayoub Qassem, a coastguard spokesman.
Operation Sophia, an EU naval mission that began last year and is authorised to seize and divert vessels suspected of being used for people smuggling, says it has contributed to the apprehension of dozens of suspects and "neutralised" more than 100 vessels.
It is now planning to expand to include coastguard training and enforcement of a U.N. arms embargo, and Britain says it intends to deploy a warship to the southern Mediterranean to help.
But without a request from Libya's unity government, those missions are unable to gain entry to Libyan waters. Even if they could, it is unclear where migrants they picked up could be sent, given the risk of mistreatment in Libya.
A British parliamentary report published last month found Operation Sophia had so far failed to disrupt smuggling "in any meaningful way".
"The arrests made to date have been of low-level targets, while the destruction of vessels has simply caused the smugglers to shift from using wooden boats to rubber dinghies, which are even more unsafe," the report said.
(Writing by Aidan Lewis, editing by Peter Millership)
Our Standards: The Thomson Reuters Trust Principles.