The move by the UN High Commissioner for Human Rights came after Nepalese security forces shot dead four protesters over the weekend in fresh wave of violence in the volatile Terai region.
The southern plains are the scene of an ongoing blockade of key border trade points with India and has witnessed sporadic violence for over 100 days now, leading to the death of nearly 50 people.
The global rights body had information of at least four persons being killed in clashes over the weekend, and had also received reports of stone-pelting by protesters at the police, who retaliated with live ammunition, she said.
Shamdasani called on protesters - led by Indian-origin Madhesis who are inhabitants of the Terai region - to also renounce violence. The Madhesis are protesting division of their homeland in the new Constitution and are pressing for better representation in the Parliament.
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"We have been urging Nepalese authorities to ensure that (the) existing standards on the appropriate use of force are fully respected.
"Unfortunately, reports suggest that these steps do not appear to have been taken," she said.
The latest killings sparked condemnation from India which said it was "distressed" at the loss of lives in the police firing.