The United Nations has called for an end to violence in Nepal following a series of deadly clashes between police and demonstrators protesting against a proposed constitution.

The spokesman for UN Secretary General Ban Ki-moon, Stephane Dujarric, said in a statement on Thursday that the UN chief was "concerned about reports of violence in recent weeks and saddened by the loss of life."

Ban has reiterated that the ongoing crisis in Nepal should only be resolved through dialogue, Dujarric said. "He (Ban) urges all to refrain from the use of force, denounce violence in all forms and engage in dialogue."

Meanwhile, an international NGO has also urged major Nepalese political parties to "recognize the depth of discontent and the fundamental challenge this poses to the legitimacy of the proposed constitution."

"A botched solution risks entrenching communal polarization in society and radicalizing groups that feel their concerns were not seriously considered," the International Crisis Group (ICG) said in a statement on Thursday. 

The new national constitution aims to restructure Nepal as a federal state made up of seven provinces, and draw a line under the issue of a decade-long civil war that ended in 2006.

A series of protests has rocked parts of Nepal, signaling that the proposed constitution is not likely to allay the concerns of the country’s many marginalized groups. Deadly clashes between police and anti-charter protesters have claimed the lives of nearly 20 people across Nepal in recent days.

Local administrations in Nepal have already imposed overnight curfews in several districts to prevent street demonstrations.  

In early June, Nepal’s major rival political parties reached a historic agreement to end years of deadlock on a new constitution for the country. Four parties – two ruling and two opposition – inked the historic deal, which also called for the settlement of the issue of federalism, the main bone of contention among the political parties.

Regional parties have long pushed for the creation of new provinces, a move which would favor historically marginalized communities. Other parties see the demand as divisive and threatening national unity.

Experts say the agreement leaves the issue of provincial borders unresolved, which itself could create future problems.