Greece is struggling to reduce overcrowding at refugee camps on its islands, even though the flow of people is less than last year.
Thousands of refugees and migrants were forced to flee on Monday when their overcrowded camp on the Greek island of Lesbos was ravaged by fire and police are investigating arson.
No casualties were reported but tents at the Moria camp were "almost entirely destroyed" and containers that provide additional accommodation and health and registration services were damaged, a police source in Athens told AFP.
Firefighters were prevented from tackling the fire early on by clashes that broke out among different communities in the camp, reports said.
"Between 3,000 and 4,000 migrants fled the camp of Moria," to the surrounding fields, with strong winds fanning the flames also hampering firefighters, the police source said.
However, once inside the firefighters were able to bring the fire under control.
Police were sent out after the refugees and were in the process of returning them to the camp, the officer added.
The officer said there was "no doubt" that the fire had been set on purpose from inside.
Some 150 minors housed at the camp were evacuated to a children's village on the island, the officer said.
Earlier on Monday, tensions rose in Moria owing to a rumour that refugees were about to be deported en masse to Turkey, state agency ANA reported.
But a strong police presence at the camp had initially calmed tempers, the officer said.
Another two fires broke out in the olive groves near Moria but were brought under control before the third blaze erupted at the camp.
There are now more than 60,000 refugees and migrants in Greece, most of them seeking to travel to Germany and other affluent EU countries.
But they are unable to do so after several eastern European and Balkan states shut their borders earlier this year.
Greece is struggling to reduce overcrowding at refugee camps on its islands, even though the flow of people is less than last year, and the slow processing of asylum requests is adding to frustrations that sometimes spark violence among residents.
Human rights groups have repeatedly criticised the condition of Greek camps, pointing to overcrowding and unsanitary living conditions.
The situation is particularly acute on Lesbos and other eastern Aegean islands facing Turkey, where most of the migrants land and are held for registration.
Island residents have also staged protests to demand the transportation of the refugees to the mainland.
The procedure is part of an EU-Turkey deal designed to limit the flow of refugees and migrants to Greece's shores.
According to government data, there are more than 13,000 people on five islands in facilities built to house fewer than 8,000.
Most of them are Syrian refugees fleeing civil war, in addition to Iraqis, Afghans, Pakistanis and others from the Indian subcontinent and Africa considered to be economic migrants, and as such not automatically entitled to asylum in Europe.
On Lesbos itself there are in excess of 5,600 people, over 2,000 more than the nominal capacity of the camps.
Brawls often take place, with many desperate to avoid being returned to Turkey or their home countries after spending a small fortune and risking their lives trying to escape poverty and persecution.
The fire comes as UN member states on Monday promised to try to improve the plight of millions of refugees around the world.
Speaking at the first UN refugee summit in New York, Greek Prime Minister Alexis Tsipras warned that failure to confront the refugee crisis would unleash xenophobia.
"If we fail to support this, the political repercussions will be felt not only in Greece but everywhere," he said.
More than 850,000 refugees and migrants arrived on the Greek islands last year, many after risking their lives in unseaworthy boats and dinghies.