US Secretary of State John Kerry called Burundi's election this week "deeply flawed" and urged President Pierre Nkurunziza to hold a "meaningful, serious" dialogue with the African country's opposition, the State Department said.
Nkurunziza won a third term in Tuesday's election, which was boycotted by the opposition. Rivals accused him of violating the constitution by running for another five years in office.
The election commission said on Friday that the president, who cited a court ruling saying he could run again, had secured 73 percent of the vote.
Nkurunziza's re-election bid has plunged Burundi into its biggest crisis since an ethnically charged civil war ended in 2005. Dozens of people have been killed in weeks of protests and more than 170,000 have fled to refugee camps in neighbouring states.
"It is imperative that the government of Burundi re-engage in a meaningful, serious dialogue with opposition and civil society leaders to reach a consensus on the way forward," Kerry said in a statement on Friday, declaring the vote "deeply flawed".
The ruling CNDD-FDD party dismissed US criticism of the election that came even before voting began. Party chief Pascal Nyabenda said US officials' remarks were "negative and nihilistic" and implied "the Burundian people cannot rule themselves."
Weeks of talks between the government, opposition parties, civil society groups and others failed to resolve differences.
Leading opposition politician, Agathon Rwasa, who boycotted the vote, has called for a government of national unity. The government has said it was open to the idea but ruled out conditions such as shortening the president's term.
The European Union and African Union did not send observers to the vote because they said the whole process was not credible.
Neighbouring east African nations, which did send observers, said voting was generally calm on Tuesday but the process "fell short of the principles and standards for holding free, fair, peaceful, transparent and credible elections."
It cited issues such as violence that preceded voting and security worries among the population, restrictions on the media and a boycott of the voting by opposition parties. The African states also called for dialogue.
Opponents stayed away from the June 29 parliamentary polls, which Nkurunziza's CNDD-FDD party also won, and Tuesday's presidential vote.