Thousands of people fled a migrant camp on the Greek island of Lesbos on Monday night after fire ripped through tents and destroyed containers during violence among residents, police said.

UNHCR, the United Nations refugee agency, said the fire was linked in part to poor living conditions and a prevailing sense of uncertainty among many. A police official in Athens said two riot police squads were being deployed to the island.

"Last night's fires ... symbolize the shortcomings of the European response to the refugee crisis," said Panos Navrozidis, UNHCR's Greece director.

It was not clear what caused the blaze, but Greek media said clashes had erupted following a rumour that hundreds of people would be deported.

A police official on Lesbos said nearly 60 percent of the Moria camp was destroyed in the fire.

Tensions have boiled over at overcrowded camps on Greece's islands as the slow processing of asylum requests adds to frustration over difficult living conditions there.

On Lesbos, tents and containers used for housing and social provision had burned down. Nearly everyone, including unaccompanied children, had evacuated the site, the police official said on condition of anonymity. Many later returned.

Roland Schoenbauer, a spokesman in Greece for the UNHCR, said people were "sick of waiting" in the camps.

"They don't know when their asylum claims will be processed, some people feel they don't have enough information," he said.

OVERCROWDING

Panos Navrozidis, UNHCR Greece director, said the camp has been operating at over-capacity for months, with people crammed into the facility with limited access to water, and in conditions that do not meet humanitarian standards.

Navrozidis criticised the system to process claims as "opaque and inconsistent" and said preferential treatment based on nationality led to tensions within the community.

More than 5,700 refugees and migrants are on Lesbos, stranded there by a European Union deal with Turkey preventing them going beyond the island until their asylum claims are processed. Those who do not qualify will be deported to Turkey.

Thousands have applied for asylum and the wait is long, ranging from weeks to months. Just over 500 people have been deported to Turkey since March but none of those who have requested asylum were among those, Greece says.

Despite a slowdown in arrivals from Turkey compared to last year, more than 13,500 migrants and refugees are now living on eastern Aegean islands, nearly double a capacity of 7,450.

"The situation is difficult," Christiana Kalogirou, prefect of the north Aegean region, told Greek TV. "There is a great need for decongestion of the islands ... in the future things could become much more difficult," she said.