Greek Prime Minister Alexis Tsipras has asked Europe to help in handling tens of thousands of refugees landing on its shores, drawing sharp criticism from the UN refugee agency.
The UNHCR warned on Friday that migrants landing in Greece were facing "shameful" conditions.
"It's total chaos on the islands," Vincent Cochetel, head of UNHCR's Europe division, said describing desperate, exhausted people, including women, children and unaccompanied minors, searching for food, water, shelter and information about how to proceed.
Some 124,000 people, almost all of them fleeing war and persecution in Syria, Afghanistan and Iraq, have come ashore since the beginning of the year - a 750-percent increase from the same period last year, the UN agency said.
Boatloads of migrants arriving every day had triggered a "humanitarian crisis within the economic crisis," Tsipras earlier said.
Migrants in Greece: The struggle for a better life
The influx has piled pressure on Greece's services at a time when its own citizens are struggling with harsh cuts and its government is negotiating with the European Union and the International Monetary Fund for fresh loans to stave off economic collapse.
The UN condemnation came as thousands of migrants landed Mediterranean islands with little help from the host country.
"The level of suffering we have seen on the islands is unbearable. People arrive thinking they are in the European Union. What we have seen was not anything acceptable in terms of standards of treatment," Cochetel said after visiting the Greek islands of Lesbos, Kos and Chios.
"I have never seen a situation like that. This is the European Union and this is totally shameful," he added.
At a makeshift refugee centre at Kara Tepe, a hilltop about 5km north of Lesbos island's main town of Mytilene, about 50 white tents provided by the local council struggled to accommodate the waves of people coming in daily.
Rubbish littered the area and locals said 16 toilets were frequently blocked despite attempts by authorities to keep the area clean.
Up to 10 people could be seen sharing one of the tents, while others lay on pieces of cardboard, jostling for space under the shade of olive trees in sweltering heat.
"The government had battles on plenty of fronts and probably could not give as much attention to the problem," the island's mayor Spiros Galinos told Reuters.
EU leaders have pledged to relocate 16,000 migrants over two years, which Cochetel called "far too little and too late".
Britain has said it will not participate. It is currently struggling with its own crisis as thousands of migrants seek to enter via the Channel Tunnel.
A senior spokesperson of the UNHCR told Al Jazeera on Thursday that the refugee crisis in Europe is a crisis not because of the number of refugees, but because of Europe’s failure to respond to it in a coordinated fashion.
"In our view, European countries need to work together rather than point fingers at each other. In order to deal with this situation, Europe should open more legal ways for refugees to come," William Spindler, senior spokesperson of the UNHCR, told Al Jazeera.
The UNHCR told Al Jazeera that more than 224,000 migrants and refugees had arrived in Europe in the first seven months of the 2015, exceeding the 219,000 that had made their way to the continent a year ago.