The United Nations has been forced to consider establishing refugee holding centres in north Africa and the Middle East due to the spiralling numbers of migrants attempting perilous journeys across the Mediterranean in a desperate effort to reach Europe.

The UN's refugee agency, UNHCR, has said for the first time that the large-scale processing of migrants and refugees outside Europe, in countries such as Egypt, Libya or Sudan, may be necessary as frontline authorities claim they have been abandoned by Brussels in the face of a "colossal humanitarian catastrophe".

Hundreds of thousands of people are preparing to make treacherous crossings on unseaworthy vessels from the north African coast to Greece and Italy as this summer's "boat season" gets under way, say officials. Figures for the first few months of this year already show a dramatic increase on previous years.

UNHCR's European director, Vincent Cochetel, told the Guardian: "We would not be totally against external processing if certain safeguards were in place: the right to appeal, fair process, the right to remain while appeals take place."

The EU had not found effective mechanisms to prevent migrants dying at sea, he said. Instead of focusing on ever tougher border controls, the EU needed to establish safe routes.

Campaigners for refugee rights have hitherto rejected the idea of large processing camps outside Europe, fearing refugees would be at the mercy of states with poor records on human rights and justice.

"There's no way that could work," said Judith Sutherland of Human Rights Watch (HRW). "In theory, HRW doesn't have a problem with creating channels of access to asylum in the EU from outside [but] you can't imagine [the right] conditions being met in Libya today, or indeed Egypt or Morocco."

Greece, which currently holds the EU presidency, is also pressing for the establishment of holding centres in north Africa and the Middle East in order to process refugees and migrants before they reach European soil. In addition, the Greek government is calling for an international seaborne taskforce to patrol the Mediterranean in an attempt to stem the flow of migrants. Greece will table the proposals at an EU summit next month, according to senior government officials in Athens.

"The shaping of a comprehensive immigration policy is one of the main priorities of [the Greek presidency], as well as the Italian presidency, which follows ours," Greece's deputy prime minister, Evangelos Venizelos, said.

"The Mediterranean countries of the EU are working together closely. But the Greek coastline is longer than the total coastline of the other member states. Without a substantial reassessment of [policies] … we cannot confront the crisis."

Miltiadis Varvitsiotis, Greece's merchant marine minister, said processing offices should be set up in Syria or Turkey "to examine who is eligible, or not, before people get on boats, put their lives in danger and trespass our borders".

Migrants who survived the capsizing of their boat near the Greek island of Farmakonissi arrive at Piraeus, Athens, in January earlier this year. Photograph: Louisa Gouliamaki/AFP/Getty Images

Italy has pledged to force the migrant issue to the top of the EU agenda when it takes over the presidency in July. "During the European presidency, Europe will not see an Italy banging its fist on the table, but an Italy that overturns the table," interior minister Angelino Alfano said last week.

The shift in the UNHCR's position – and the growing clamour from Greece and Italy for action – comes as the latest figures show a rapidly accelerating problem. About 42,000 people have attempted to cross the Mediterranean to Italy so far this year, according to the EU border agency, Frontex. Last year there were 3,362 arrivals by the end of April, said UNHCR.

Last week, more than 1,000 migrants stormed a razor-wire fence at Melilla, a Spanish enclave in Morocco, in order to reach European territory, 400 of whom breached the barrier. The same day, French riot police bulldozed three camps holding hundreds of refugees in the port of Calais.

The Greek merchant marine ministry said 15,000 undocumented migrants attempted to enter Europe via sea route to Greece last year, requiring a third of the 7,000-strong Greek coastguard to be redeployed to the eastern Aegean.

The numbers have been swelled by large numbers of Syrian refugees seeking to escape the civil war, which has displaced millions of people in the past three years. Some 7,000 refugees from Syria arrived on the Italian coast in the first nine months of 2013, compared with 350 in the whole of 2012. They are the second largest group to arrive by sea in Italy this year, after Eritreans. Most making boat journeys to Greece are Syrian refugees, including women and children.

Since the Greek government erected a 10.5km barbed-wire fence along its border with Turkey, 90% of illegal migration has been channelled through the eastern Aegean, with coastguards engaged in a daily battle with human traffickers.

"It's clear that the [Greek] fence … has rerouted the flow to the Aegean," said HRW's Sutherland. Another fence, planned for the Turkish-Bulgarian border, is likely to have a similar impact. "The sealing of that border will lead to even more sea crossings," she said.

In recent weeks there have been dozens of deaths in the Mediterranean and Aegean seas between Turkey and Greece. The Italian Red Cross has called for a "humanitarian corridor" to be opened for people fleeing war and famine.

In a statement condemning the EU's inaction, Amnesty International said: "With virtually no safe and legal routes into Europe, people are increasingly pushed into the hands of smugglers and traffickers, and are forced to risk their lives on unseaworthy vessels."

An airport hangar in Lampedusa contains the bodies of more than 300 migrants who drowned trying to cross the Mediterranean in October 2013. Photograph: Roberto Salomone/EPA

At least a third of those heading for Greece end up having to be rescued. Many boats are in poor condition and are overcrowded. In addition, "migrants travelling in small inflatables have explicit instructions from traffickers to sink boats once they see the Hellenic coastguard so they can be picked up in EU waters," Varvitsiotis said.

"We can't deal with this problem forever alone. Europe needs to have an equal burden policy."

Sicily has borne the brunt of illegal migrants heading for Italy since the temporary closure of a reception centre on the smaller island of Lampedusa, and local authorities say they feel abandoned by Brussels and Rome. They predict a continuing rise in arrivals over the summer.

"If strong action isn't taken, it will be a disaster," said Enzo Bianco, the centre-left mayor of Catania and former Italian interior minister. "Either there is a strong initiative by the Italian government and by the EU, or we will be facing a real disaster of colossal proportions. If we're in a crisis with 50,000 arrivals, imagine what will happen if there are 500,000-600,000."

At a funeral in Catania last week for 17 migrants who died off the coast of Libya, Bianco condemned Europe's "deafening silence": "Faced with a looming, colossal humanitarian catastrophe, with almost 800,000 people on the African coast ready to cross the Mediterranean … Faced with these coffins, Europe must choose [whether to] bury our consciences of civilised men along with them."

After 366 migrants died off the coast of Lampedusa last autumn, the European Commission set up Eurosur, a surveillance operation aimed at reacting more quickly to boats in distress. This was strengthened by the Italian search and rescue operation Mare Nostrum, which has so far rescued 30,000 people from the sea at a cost of more than €9m (£7.3m) a month.

Italy says the headquarters of the European border authority Frontex should move from Poland to Sicily to deal with the growing crisis. "Europe is distracted. But Europe can't pretend it's nothing. The problem exists … We need resources," said Luigi Ammatunna, the mayor of the small town of Pozzallo on Sicily's south-eastern coast, which has received 8,500 migrants this year.

The cash-strapped Greek government said it spent €65m to protect the eastern seafront last year, with only €2m contributed by the EU.

Rising numbers of refugees and migrants have fuelled support for rightwing politicians in Greece and Italy. The neo-fascist Golden Dawn party, which has focused on the issue, pulled off a surprisingly strong performance in local and European elections in Greece. The leader of the rightwing xenophobic Northern League party in Italy, Matteo Salvini, flew to Sicily last month to demand an end to the Mare Nostrum operation.

Alfano, the Italian interior minister, adopted increasingly provocative rhetoric with Brussels during the European election campaign, threatening to defy EU asylum rules and "just let them [asylum seekers] go" out of Italy to other countries.

Last week, he warned Brussels to shoulder the €300,000-a-day cost of Mare Nostrum, or the Italian government would deduct it from its EU contributions. Italy, he said, could not "become the prison of refugees who want to go to northern Europe".

Meanwhile, refugees in Sicily are being housed in makeshift accommodation, such as schools and sports halls, with inadequate reception and processing facilities. The arrival of large numbers of migrants, including many unaccompanied children, "creates problems for us – not because we don't want them, but because we want to provide a reception worthy of the name", said Ammatunna. "Receiving these people, then treating them badly, not giving them a proper welcome, not being able to give the services that we would like to be able to give them – this we do not want."