The state of migrants stuck on Greek Islands is getting desperate and teeters "on the edge of catastrophe", the Council of Europe Commissioner for Human Rights warned on Thursday.

Addressing reporters after a five-day tour, Dunja Mijatovic said the situation in migrant camps in the Greek Aegean islands has become "explosive" and urgent measures need to be taken to address the appalling conditions in which thousands of asylum-seekers are living.

Mijatovic visited camps on the islands of Lesbos and Samos and in Corinth.

"There is a crying lack of medical care and sanitation in the vastly overcrowded camps I have visited. People queue for hours to get food and to go to bathrooms, when they exist.

"On Samos, families are chipping away rocks to make some space on steep hillsides to set up their makeshift shelters, often made by trees they cut themselves. This no longer has anything to do with the reception of asylum seekers. This has become a struggle for survival," she added.

Greece, a popular entry point for people seeking refuge in Europe, hosts over 70,000 migrants on its soil.

More than 34,000 people are stranded in camps on Greek islands, despite frequent relocation to the mainland.

A new conservative government, which came to power in July, has vowed to introduce more stringent asylum rules, increase sea patrols to deter migrants arriving by boats and send 10,000 people back to Turkey by 2020.

The Greek parliament is on the cusp of adopting a new migration law on Thursday that received criticism from both Mijatovic and human rights groups.

The bill has been denounced for stricter asylum rules, narrowing the definition of family, delays to the right to work and further convoluting the process of recognizing torture victims.