Guterres said the opposition and civil society groups must be included in any decision to change the constitution to avoid a flareup of conflict in Burundi.
“While it is the sovereign right of Burundi to amend its constitution, I strongly believe that such an important undertaking must be conducted in an inclusive manner that seeks to achieve a maximum possible participation and consensus among the key political stakeholders,” said the report sent to the Security Council on Monday.
The council is due to discuss the report on February 26.
In power since 2005, Nkurunziza ran for a third term and was re-elected in 2015 despite provisions of the Arusha peace accord that set a limit of two five-year presidential terms.
The decision to run for a third term sparked violence in Burundi that has left hundreds dead, and sent more than 400,000 Burundians fleeing across borders, mostly in Tanzania.
Nkurunziza has launched a campaign to hold referendum on a new constitution probably in May 2018 that would set a limit of two seven-year presidential terms.
Several opposition leaders have either boycotted the talks on the constitutional changes or raised questions about their legality.
– Peace deal jeopardized –
The constitutional changes would modify the ethnic quotas between Hutus and Tutsis that were outlined in the hard-won Arusha peace accord that ended the civil war.
“Durable peace comes with addressing the underlying root causes of the crisis, not by jeopardizing the foundations of relative normalcy, such as the Arusha Agreement, which brought a decade of peace to the country,” said Guterres.
The UN chief said he was “deeply concerned” that talks between the government and the opposition remain deadlocked, and said it was crucial that all sides “most especially the government” engage in a dialogue.
Ugandan President Yoweri Museveni and former Tanzanian president Benjamin Mkapa are leading a regional effort to end the crisis in Burundi, but the report said the mediators have made little significant progress.
Burundi’s UN ambassador Albert Shingiro told AFP he had not read the report, but added that “any criticism of the planned constitutional amendments in Burundi is a violation of state sovereignty in line with the UN charter.”
Shingiro complained that the United Nations “has never raised the same concerns when there is a review of the constitution in the region or elsewhere in the world.”
Rwandan President Paul Kagame won a third term in office last year after constitutional changes, and there is pressure on President Joseph Kabila in the Democratic Republic of Congo to step down after elections in December.
The UN rights office continues to receive allegations of killings, enforced disappearances and torture, primarily by Burundi’s state security, the report said.
A UN commission of inquiry set up in 2016 has found that crimes against humanity had been committed in Burundi.
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