Migrants at a camp on the Greek island of Lesbos on Monday hurled stones at police and torched temporary offices used by asylum officials in an hour-long protest over conditions.

Jose Carreira, executive director of the European Asylum Support Office (EASO), said at least four shipping containers where interviews were conducted were entirely destroyed, and three more were damaged.

‘Incidents have occurred in the past but this is the most serious one,’ Carreira told AFP.

‘We are looking into ways of guaranteeing that this might not happen again,’ said Carreira, who is in Athens for meetings with Greece's immigration ministry.

No-one was hurt in the incident as the blaze was quickly brought under control by firefighters, but Carreira said it could be days before asylum interviews can resume.

Around 70 migrants took part in the protest, most of them from Pakistan and Bangladesh, a local police source said. Police made a dozen arrests.

The incident took place at the overcrowded Moria camp, one of five centres on Greek Aegean islands, which have borne the brunt of Europe's migrant crisis.

More than 15,000 migrants are being held on the Aegean islands, pending their return to Turkey under an EU-Turkish agreement reached in March this year.

Processing has been held up because the vast majority have filed for political asylum.

Part of Moria, which has a capacity for 3,500 people but currently houses more than 5,000, was badly damaged in a fire during clashes last month between migrants and police.

Nearly 66,000 refugees and migrants are currently stranded in Greece, according to official figures.

In Bulgaria meanwhile, some 300 Afghan migrants protested on Monday in the country's largest migrant reception centre, near the Turkish border, according to a human rights group.

‘They protested against the fact that they are being kept in Bulgaria against their will and demanded to be able to leave towards Serbia and then (western) Europe,’ said Iliana Savova, a spokeswoman for the Bulgarian Helsinki Committee for Human Rights, adding that around 300 migrants took part in the demonstration.

Bulgaria's interior ministry confirmed ‘tensions’ at the site, which is home to around 3,800 migrants -- around half of whom are Afghans.

Some 13,000 illegal migrants remain stranded inside Bulgaria, after other transit countries along the western Balkan route shut their borders earlier this year.

And close to 50 migrants ended a brief hunger strike Monday in an immigration detention centre in Barcelona, police said, just days after a riot took place in a similar facility in Madrid.