New York-based Human Rights Watch on Thursday urged Nepal to investigate allegations of rights violations during months of deadly protests after the Himalayan republic adopted its first democratic constitution last year.

In response, Nepali authorities told Reuters they were serious about protecting human rights and had put police on alert to prevent infringements.

Nepal promulgated the charter last September, sparking protests that killed more than 50 people, mostly demonstrators from its Madhesi ethnic minority, who feared the charter would weaken their role in the power structure.

Clashes between police and protesters, worried that the redrawing of southern provincial borders would divide the community, also left many injured, amid charges of abuses by both sides.

In a letter, Human Rights Watch urged Prime Minister K.P. Oli to ensure accountability for violations as well as appropriate disciplinary and legal action in cases of excessive use of force, arbitrary arrests, and torture.

"Despite endless promises of reform, impunity remains the norm in Nepal," Brad Adams, Asia director at the non-governmental organisation, said in a statement.

"Accountability for any abuses by both protesters and police are important, and unfortunately at the moment it looks as if alleged abuses by the police will simply be forgotten."

HRW said police were reportedly refusing to register First Information Reports, or criminal complaints making it possible for an investigation to go ahead, filed on behalf of victims.

An interior ministry official told Reuters that complaints of abuse must be accompanied by proof before police can act on them, but did not say what evidence was needed.

"We are serious about protecting human rights and had ordered the police to remain alert against any abuses during the protests," said Yadav Koirala, the official.