By Umberto Bacchi

LONDON (Thomson Reuters Foundation) - The top United Nations human rights official said on Wednesday the conviction in Paraguay of 11 landless farmers accused of being involved in deadly clashes with police four years ago was "deeply troubling".

In June 2012, 17 people, including six police officers, were killed as security forces moved in to evict about 60 farmers who were occupying 2,000 hectares of land in Curuguaty, eastern Paraguay.

Last week, a court in Asuncion sentenced 11 farmers to up to 35 years in jail on charges ranging from homicide to criminal association over the confrontation, which highlighted tensions over land rights in the South American country.

U.N. High Commissioner for Human Rights Zeid Ra'ad Al Hussein said he was concerned by reports from rights groups that the investigation into the incident was one-sided and marred by irregularities.

"I'm also deeply concerned by the fact that, up to now, the deaths of 11 peasants, killed in the same incident, have not been investigated by Paraguayan authorities, nor have the allegations that some were summarily executed after being subjected to torture and other human rights violations," Zeid said in a statement.

Farmers in Paraguay have long demanded a redistribution of agricultural land, 80 percent of which is held by just 2.5 percent of the population, according to a 2008 census.

Zeid also urged the authorities to "guarantee that victims have access to justice" and repeated calls for an independent and impartial probe.

Campaign groups say inequalities in land distribution in Paraguay stem back to the dictatorship of General Alfredo Stroessner, who gave away titles to about eight million hectares of state-owned land to members of the military and political elite.

Zeid said there was urgent need for Paraguay to speed up land reform as land ownership remains one of the most "critical" issues in the country and a major source of social tensions.

(Reporting by Umberto Bacchi @UmbertoBacchi, Editing by Katie Nguyen.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, property rights and climate change. Visit http://news.trust.org)