The failure by Somalia authorities to respond to election-related killings last year reflects "the lack of functioning justice institutions throughout the country," two United Nations agencies said on Monday.

Some 44 clan elders and electoral delegates were killed in connection with a parliamentary and presidential selection held in late 2016 and early 2017, the UN human rights agency and the UN assistance mission in Somalia noted in a joint report. In only two of those cases were arrests made and prosecutions carried out.

"Lack of independence and impartiality, limited capacity and resources, weak law enforcement and a deep-rooted public perception of corruption within the judiciary all contribute to the lack of an effective justice system," the report stated.

These critical findings are the latest to highlight the poor governance and pervasive insecurity that continue to plague Somalia 11 years after the African Union dispatched troops to stabilise the country.

Funds from the US and EU spent to professionalise Somalia's army and strengthen its political institutions likewise appear to have had little positive effect.

Violence from Al Shabaab insurgents and clan militias may actually be worsening, the new report suggests. Explosions of improvised devices and "complex attacks" by militants increased from 270 in 2016 to 413 last year, according to the UN's Somalia mission. Deaths resulting from these incidents totalled 481 last year, up from 296 in 2016.

Nearly 1,400 civilians were killed in Somalia last year, the agencies said.

Al Shabaab was responsible for 62 per cent of those deaths, while clan militias accounted for 13 per cent. Eight per cent of the killings were attributed to Somali government forces and three per cent to African Union troops. The remaining 14 per cent were victims of other groups or unidentified parties.

The voting itself was organised as an exercise in "clan distribution" of parliamentary seats rather than as an expression of the democratic principle of one person, one vote, the report says.

Only 14,000 of Somalia's 14 million people were permitted to take part in what the UN characterised as "a political process with electoral features."