Conditions in two of Haiti's main jails are "cruel, inhuman and degrading", with severe overcrowding after prisons were damaged in an earthquake in January, a UN human rights expert warned Wednesday.

The independent expert on the situation of human rights in Haiti, Michel Forst, visited the National Penitentiary and the prison of Cayes April 21 to May 1.

"Both places are overcrowded with detainees living in cruel, inhuman, and degrading conditions, in the meaning of the (UN) convention against torture," he said in a report to the UN Human Rights Council.

Part of the National Penitentiary is in danger of collapsing and detainees are kept in one building "where conditions are even more severe than before the earthquake", the report added.

Forst cited International Committee of the Red Cross (ICRC) estimates that each prisoner had on average 0.3 square metres (3.2 square feet) of floor space.

Forst also called for a "serious and impartial" investigation after about 10 prisoners at Cayes were shot dead when they tried to escape a week after the January 12 earthquake.

Relatives and other prisoners said they were shot by police and their bodies were buried in a mass grave.

Forst said some of about 5,000 prisoners who escaped when the quake shattered their jails returned of their own volition, although he gave no number.

However, gang chiefs were still out, adding to insecurity and the decline in an already "precarious" human rights situation, he added.

Some information indicated that complaints to police for violent crime in Haiti had grown by 150 percent, the report said, with gangs acting with impunity.

The earthquake leveled much of the capital Port-au-Prince, killing more than 250,000 people.