Louise Arbour said in a statement on Thursday, after the end of a four-month ceasefire: "It is a tragedy for the people of Nepal that full-scale armed conflict may now resume.
"But there need not and must not be the same gross violations of international humanitarian law and human rights standards that have been perpetrated during previous phases of the conflict."
Arbour referred to a history of executions, abductions, attacks on buses, indiscriminate bombing of civilian areas and widespread torture. Children have been killed, recruited, used as informers, and arbitrarily detained and beaten.
She said that Nepal had ratified the Geneva Conventions on the treatment of prisoners, as well as other human rights treaties, and the Nepalese Communist Party had made general commitments to respect international humanitarian law.
Both sides should "declare publicly their acceptance of all that these principles, and to explain to their cadres their responsibility to respect them in practice", Arbour said.
"Those on either side of the conflict who commit violations must be held accountable - not only the perpetrators but also those in command of forces which commit such acts may be subject to individual criminal responsibility before a court of law."
On Thursday, three police officers were killed by the rebels and at least 11 people were wounded in the worst day of violence since the ceasefire ended this week, police and witnesses said.