The 10 independent experts also "unreservedly condemned" Monday's Tamil Tiger suicide attack that killed 28 people outside a refugee camp in the northern area of Visuamadu, in a joint statement released by the United Nations.
They warned of "disturbing reports" of torture, extrajudicial killings and enforced disappearances throughout the country.
The experts on freedom of expression, disappearances, arbitrary detention, health, independence of judges and lawyers, executions, torture, human rights defenders, housing and food, called for "thorough reforms of the general system of governance.
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Margaret Sekaggya, special rapporteur on the situation of human rights defenders, said the situation had worsened over the past year.
"A climate of fear and intimidation reigns over those defending human rights, especially over journalists and lawyers," she added, underlining that serious and fatal attacks on journalists were "now a common occurrence.
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Despite the "severity of the abuses