UN official Radhouane Nouicer's Sudan visit, which began Saturday, comes more than a month after civilian and military factions agreed on an initial step in a two-phase political process aimed at restoring stability.
Nouicer, a Tunisian diplomat, met with coup leader General al-Burhan and his deputy Mohamed Hamdan Daglo, as well as other senior Sudanese officials.
The UN human rights expert urged authorities to "hold to account" officials who had abused their powers and called for "a clear roadmap for security sector reform".
"The immunity from prosecution of members of the security forces implicated in human rights violations must be lifted," he told a news conference in Khartoum.
He cited violations documented by the United Nations since the military takeover, including torture, sexual violence, and arbitrary arrests and detentions.
Nouicer also called on authorities to allow the work of civil society groups who have complained about "mounting restrictions" on their activities.
The Tunisian took up the position of rights expert for Sudan in December, replacing Adama Dieng who had resigned two months prior.
The coup triggered near-weekly demonstrations which were met by a violent crackdown that killed more than 120 people, according to pro-democracy medics.
The coup has also deepened a spiraling economic crisis and heightened ethnic clashes in Sudan's remote regions, which killed around 900 people last year, according to the UN Office for the Coordination of Humanitarian Affairs.
