Separately, the 15 UN Security Council members plan to hold consultations on the crisis Tuesday.
In a preliminary report, UN observers said Thursday's vote, which saw President Pierre Nkurunziza re-elected, was marred by violence and obstacles to freedom of expression and the press.
"While election day was relatively peaceful and conducted adequately, the overall environment was not conducive for an inclusive free and credible electoral process," the report said.
It said the elections were held "in an environment of profound mistrust between opposing political camps."
"Freedom of expression, assembly and association, essential conditions for the effective exercise of the right to vote, remained severely impaired," the report said.
It faulted the state-run media for failing to provide balanced coverage to all presidential candidates.
"Nevertheless, on election day Burundians in most places went peacefully to the polls to cast their ballots," the report said.
It echoed a separate report the UN mission had already issued in June after legislative and local elections in Burundi.
The mission's latest findings were in line with those of other international observers.
Results made public Thursday by Burundi's electoral commission had Nkurunziza winning a third term as president with 69.41 percent.
His decision to run for re-election in late April plunged the country into a profound political crisis accompanied by violence that left more than 80 people dead.