Children being bitten by scorpions, rats and snakes; hundreds being forced to use a single shower; the stench of human excrement never far away; and food shortages becoming the norm. One by one, Sophie McCann lists the degradations of life for refugees detained on Lesbos, the Greek island on the frontline of a new surge of asylum seekers desperate to reach Europe.

McCann, a British advocacy manager with Médecins Sans Frontières, like other aid workers, is now raising the alarm: at least 24,000 men, women and children trapped in vastly overcrowded Aegean island camps are being subjected to conditions so harrowing they bear all the hallmarks of humanitarian catastrophe. Four years after the greatest migration crisis in modern times, there are fears history is repeating itself.

“The level of human suffering is just indescribable,” says McCann, adding that incidents of self-harm, even among toddlers, have risen sharply. “I struggle to find the right words because none can convey the sheer misery and inhumanity of a situation that in Europe is frankly unbelievable.”

The drama unfolding on Europe’s south-eastern periphery follows the abrupt rise in numbers making the dangerous sea crossing from Turkey.

With a brazenness unseen since the EU struck a controversial accord with Ankara to stem flows in 2016, smugglers are again landing boatloads of would-be asylum seekers on the beaches of Lesbos, Samos, Kos, Leros and Chios.

In July and August more than 13,000 people landed on Greek shores, more than half of all sea arrivals to the country in 2019. With the uptick continuing into September, the surge has sparked fears that Europe is on the verge of a new refugee crisis – despite numbers still being a fraction of those in 2016 when as many as 2,000 people a day were reportedly disembarking from boats.

At least a third of the newcomers are unaccompanied minors, according to the UN refugee agency. “It has happened so quickly, aid organisations feel overwhelmed,” said Sofia Malmqvist at the Athens branch of the International Federation of Red Cross and Red Crescent Societies.

“The situation is particularly acute on the islands. We are deeply concerned. There are a lot of children. What we are seeing is authorities [pursuing] short-term solutions such as moving some to the mainland and into tents, but what is going to happen in the winter?”

The influx comes amid mounting anti-refugee sentiment in Turkey on the back of the country’s economic woes and growing unemployment. In recent weeks the president, Recep Tayyip Erdoğan, has repeatedly warned that his government will be forced to “open the gates” if the EU fails to deliver €6bn in promised funds – remuneration set out in the landmark accord – and Ankara is prevented from moving ahead with plans to create a safe zone in northern Syria where refugees could be relocated.

Turkey currently hosts close to 4 million displaced Syrians with the prospect of that number escalating if yet more flee across the border from Idlib, the rebel stronghold that has become the focus of a renewed offensive by the regime of Bashar al-Assad and Damascus’ backers in Moscow.

As a result of the summer rise, Greece has overtaken Italy and Spain as the European Union’s busiest entry point for illegal migration.

In Athens, NGOs attribute the increase to the Turkish coastguard deliberately relaxing patrols. EU diplomats speak of Erdoğan’s ability to hold the bloc to ransom by “opening and closing the faucet”.

Amid fears of a populist backlash, Germany, which took in more Syrian refugees than any other EU member state in 2015, has announced it is watching the illegal sea crossings with mounting concern.

Like Greece’s new centre-right government, Berlin sees a solution in greater numbers of asylum seekers being sent back to Turkey. The latest arrivals topped the agenda when Greece’s foreign minister, Nikos Dendias, held talks in Berlin on Monday in which it was stressed it was vital that Ankara stick with the pact it has agreed with the EU.

In almost four years since the EU-Turkey accord was reached, barely 1,000 have been returned – a reflection of the human rights concerns voiced by Athens’ former leftist administration in deporting migrants back to Turkey. Under the agreement refugees have to process asylum claims on “entry point” islands before being allowed to move on.

In recent weeks Greek authorities have stepped up emergency evacuations, transferring some 1,800 people deemed to be vulnerable to camps on the mainland.

“But,” says McCann, “these cycles of emergency decongestion won’t solve the underlining problems. This is a policy-driven crisis where the EU has sought to contain and externalise the problem [of migration] to the Greek isles. The EU-Turkey deal was supposed to be a ‘temporary and extraordinary measure’ to reduce flows and open safe legal alternatives to smugglers. Instead it has created camps where people are robbed of their dignity and forced to live in horrendous conditions.”

She said that in July and August, 73 children were referred to MSF’s paediatric mental health teams in Lesbos: three had attempted to kill themselves and 17 had self-harmed.

On all Aegean islands most affected by the influx, infrastructure is now at breaking point.

Moria, the main reception centre in Lesbos, was so overcrowded last week there was no space to “even install more chemical toilets”, the camp’s exasperated governor, Yannis Balbakakis, lamented after unexpectedly announcing he was resigning from the post.

The camp is currently home to 10,400 people although it was designed to host 3,000.

The reception centre in Samos is even worse, “running seven times above capacity”, noted Stella Nanou, the UN refugee agency’s spokeswoman in Athens.

“The increase in recent sea arrivals has put additional strain on already overcrowded facilities on the islands where conditions remain dire,” she said.

“With very few other official or legal pathways out of Greece the continued arrival of asylum seekers would entail the need for an ever-expanding reception capacity which is simply unsustainable. A longer-term plan is needed from the Greek authorities, with the support of the EU to define a [burden sharing] strategy to accommodate asylum seekers and refugees.”