Greece's largest migrant camp on the island of Lesbos was gutted by a huge fire early Wednesday, leaving over 12,000 fleeing asylum seekers homeless, with reports that the blaze was started by migrants revolting against coronavirus isolation rules.

Firemen said the notorious Moria camp had been mostly destroyed but there were no known casualties so far, though a number of people were suffering minor respiratory problems from exposure to smoke.

Government spokesman Stelios Petsas said an emergency would be declared across the entire island and reports of arson were under investigation.

He warned that authorities faced a "titanic" effort to shelter asylum seekers rendered homeless by the blaze, as well as track down and isolate dozens of confirmed coronavirus infections among them.

"There are 35 positive cases and they need to be isolated... to prevent an outbreak among the local population," Petsas told state TV ERT.

Hundreds of asylum seekers attempted to flee on foot towards the port town of Mytilene but were blocked by police vehicles, while others had taken shelter in the hills surrounding the camps.

Refugee support group Stand by Me Lesvos said on Twitter it had received reports that Greek locals on the island had blocked fleeing asylum seekers from heading into a nearby village.

"The whole camp is on fire. Everything is burning. People are escaping. Their homes in Moria are gone," the organisation said.

Administration offices, a clinic inside the camp and thousands of tents and were damaged by the fire, according to local news site Lesvospost.

Nearly the entire Moria camp was on fire early Wednesday, according to an AFP photographer
Nearly the entire Moria camp was on fire early Wednesday, according to an AFP photographer AFP / Manolis LAGOUTARIS

Greek news agency ANA said the fires had started after a revolt by asylum seekers who were to be placed in isolation after testing positive for coronavirus or coming into close contact with an infected patient.

Firefighters also said they had been blocked from entering the camp to battle the blaze by a group of refugees and had asked for police assistance.

The camp has seen a spike in coronavirus patients since reporting its first case last Wednesday, with 35 confirmed infections so far.

Migrant camps on the islands have endured months of lockdowns as a result of the coronavirus pandemic, with access severely restricted.

But at Moria, the restrictions have been harder to enforce because of the large number of asylum seekers sleeping outside the camp's walls.

Rights groups including the UN refugee agency have urged Greece to reduce congestion in its migrant camps.

The Moria camp, which was built to hold fewer than 2,800 people, had been routinely criticised for its lack of hygiene and overcrowding.

Many refugees have been unable to find lodgings and jobs after leaving the camps, with housing and cash benefits recently scaled back by the government.