Reacting to the call from the opposition, a presidential official said that Nkurunziza would not oppose forming a national unity government in the event he wins the election.
"A government of national unity is not a problem for Pierre Nkurunziza, we are ready to do so," said Willy Nyamitwe, an adviser to the president.
But he rejected the idea of cutting short any new five-year mandate as "impossible."
Vote counting is under way and results are expected on Thursday in the election, which was boycotted by Burundi's opposition and civil society groups over fears of electoral fraud.
The U.S. has said the election is not credible, as it has been tainted by the government's harassment of opposition and civic groups, closing of media outlets and intimidation of voters.
About 74 percent of Burundi's 3.8 million registered voters cast their ballots, electoral commission President Pierre-Claver Ndayicariye said Wednesday.
Nkurunziza is widely expected to win a third consecutive term.
Shortly before voting started on Tuesday, a policeman and a civilian were killed amid a string of explosions and gunfire in the capital Bujumbura, the epicenter of three months of anti-government protests.
The opposition has denounced the candidacy of the incumbent president as unconstitutional and a violation of the 2006 peace deal that ended a dozen years of civil war and ethnic massacres.
The nation's constitutional court has ruled in the president's favor, maintaining he is eligible for a third term because he was chosen by legislators — and not popularly elected — for his first term.
More than 100 people have died in protests since the ruling party announced in April that Nkurunziza would run for a third term.
Independent media has been shut down and many opponents have fled — joining an exodus of more than 150,000 Burundians who fear their country may again be engulfed by widespread violence.
Al Jazeera and wire services
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