The UN voiced alarm Friday at reports that countries are failing to help migrants in distress on the Mediterranean Sea, blocking assistance by NGOs and coordinating pushbacks of their boats.

UN rights office spokesman Rupert Colville warned during a virtual press briefing that such measures "are clearly putting lives at risk".

"We are deeply concerned about recent reports of failure to assist and coordinated pushbacks of migrant boats in the central Mediterranean, which continues to be one of the deadliest migration routes in the world," he said.

More than 100,000 migrants tried to cross the Mediterranean last year with 1,200 dying in the attempt, according to the UN's International Organization for Migration.

And so far this year, more than 250 migrants have perished trying to make the perilous crossing.

Colville pointed to claims that distress calls to Maritime Rescue Coordination centres "have gone unanswered or been ignored".

"If true," he said, this "seriously calls into question the commitments of the states concerned to saving lives and respecting human rights."

He especially decried reports that Maltese authorities had asked commercial ships to push boats with migrants and refugees in distress back to the high seas.

And he lamented that humanitarian search and rescue vessels that usually patrol the central Mediterranean have been prevented from helping migrants, even as the numbers of attempted crossings surge.

More than 250 migrants have perished trying to make the perilous crossing of the Mediterranean Sea so far this year
More than 250 migrants have perished trying to make the perilous crossing of the Mediterranean Sea so far this year sea-eye.org / Cedric FETTOUCHE

During the first three months of this year, departures from war-torn Libya increased four-fold compared to the same period in 2019, Colville said, stressing that migrants and refugees embarking on such journeys were entitled to protection under international law.

"Yet, since April 9, both Italy and Malta have declared their ports 'unsafe' for disembarkation due to COVID-19," he said.

Italy, the most common destination for rescue boats, has been one of the most affected countries in the pandemic, with nearly 30,000 deaths.

As a result of port closures, at least three vessels with migrants onboard are awaiting disembarkation, Colville said.

He pointed to reports that a small group of adults, including pregnant women, and children were allowed to disembark on Thursday after the Maltese government made a concession on humanitarian grounds.

"While we welcome this effort, we call for all migrants currently being held on board these vessels to be urgently disembarked, as the conditions on merchant vessels are not suitable for long-term accommodation," he said.

He also decried that the Libyan Coast Guard continues to turn vessels back to its shores and to detain all intercepted migrants in "horrendous conditions", warning that overcrowding there also made detainees vulnerable to the novel coronavirus.

Since the summer of 2018, the European Union has tasked Libya's coastguard with coordinating search and rescue operations in a vast stretch of the Mediterranean beyond their territorial waters.

But the UN and others have long warned that it is not safe for migrants to be returned to the conflict-ravaged country.

"Libya cannot be considered to have a safe port for disembarkation," UN refugee agency spokesman Charlie Yaxley told Friday's briefing.