The UN refugee agency says people smugglers are taking greater risks to ferry their human cargo towards Europe as Libya's coastguard intercepts more and more boats carrying migrants, increasing the likelihood that those on board may die during the Mediterranean journeys.

That's one of the key findings from the latest UNHCR report about efforts to reach Europe. The report says that even though the number of crossings and deaths has plunged compared to recent years, the voyage is more deadly in percentage terms for those who venture across.

The report says 2276 people died last year while trying to cross, or one death for every 42 arrivals. This year, it's 1095 deaths, or one out of every 18 arrivals.

In June alone, the proportion hit one death for every seven arrivals.

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On the Central Mediterranean route so far this year, there have been 10 separate incidents in which 50 or more people died - most after departing from Libya. Seven of those incidents have been since June alone, UNHCR said.

"The reason the traffic has become more deadly is that the traffickers are taking more risk, because there is more surveillance exercised by the Libyan coastguards," said Vincent Cochetel, UNHCR's special envoy for the central Mediterranean.

"They are trying to cut the costs: It costs them more to keep those people here longer in their warehouses, under captivity."

Libyan authorities intercepted or rescued 18,400 people between August last year and July this year - a 38 per cent increase from the same period of 2016 and 2017. Arrivals by sea from Libya to Europe plummeted 82 per cent in those comparable periods, to 30,800 in the more recent one.

UNHCR says a growing worry these days is deaths on land by people trying to get to Libya in the first place, or getting stuck in squalid, overcrowded detention centres: Many get returned there after failing to cross by sea to Europe.

"Many [then] disappear," Cochetel said. "Many are sold to militias, and to traffickers, and people employing them without paying them."

- AP