Thousands of asylum seekers on the Greek island of Lesbos were camping on roadsides on Thursday after the country's largest camp burnt down, with officials racing to find them shelter and avert a health crisis.

Desperate families, many with young children, spent a second night in the open, some without tents or basic bedding.

Some of the homeless trekked to the nearest villages for water and other supplies.

The fire late on Tuesday at Moria camp, Greece's most notorious migrant facility, sent thousands fleeing for safety into surrounding olive groves.

While nobody was seriously hurt, the blaze destroyed the official part of the camp, which housed 4,000 people, ministers said. 

A second fire late on Wednesday destroyed most of the remaining camp where another 8,000 lived in tents and makeshift shacks around the perimeter, the migration ministry said in a statement on Thursday.

Photo: AFP

The ministry said a ferry had been sent to accommodate hundreds of people ahead of the expected arrival of European Commission vice-president Margaritis Schinas to inspect conditions on the island.

"Today all necessary actions will be taken to immediately shelter families and vulnerable persons to begin with," the ministry said.

Two Greek navy vessels would provide additional sleeping space, the ministry said.

The government also said it had sent three flights to remove 406 unaccompanied minors from the island.

The children have been tested for the novel coronavirus, and will be housed in "safe" facilities in northern Greece.

Migration minister Notis Mitarachi on Wednesday said asylum seekers had started the fire because of quarantine measures imposed after 35 people at the camp tested positive for coronavirus.

Officials have declared a four-month emergency on Lesbos and flown in additional riot police.

Greece's public health authority EODY said eight of the 35 positive cases had been located and isolated along with a significant number of their close contacts.

Medical staff from the World Health Organization are expected in Lesbos on Thursday to begin tests on asylum seekers and locals, EODY chief Panagiotis Arkoumaneas told reporters on Wednesday.

European countries from Germany to Norway - along with EU chiefs - have responded with offers of help as calls intensify for urgent reform of the bloc's asylum system.

Since becoming one of the main gateways into Europe for migrants and asylum-seekers in 2015, Greece has built dozens of detention centres around the country.

But with other European nations accepting only a small trickle of refugees, thousands remain trapped in the Greek camps in usually dismal health conditions.

Greece's conservative government has also toughened its asylum restrictions, slashing cash benefits and accommodation provisions to discourage further migration.