“The United States does not believe that the results announced today represent a credible expression of the will of the Zimbabwean people," US Secretary of State John Kerry said in a statement on Saturday.
"There were irregularities in the provision and composition of the voters roll .The parties had unequal access to the state media. The security sector did not safeguard the electoral process on the even-handed basis."
The Zimbabwe Electoral Commission (ZEC) announced on Saturday that Mugabe won the presidential election with 61.09% of the vote while his rival, opposition leader Morgan Tsvangirai, got only 33.94%, according to final results.
It also confirmed that Mugabe's ZANUPF won 158 seats in the 210-seat National Assembly.
Tsvangirai's Movement of Democratic Change (MDC) won 50 seats, while two seats went to independent candidates.
The opposition has refused to accept the results, vowing to challenge them in court.
Britain also said it had “grave concerns” about the conduct of the Zimbabwean election.
“The irregularities in the lead up to the election and on Election Day itself, reported by the observer missions and in contravention of the South African Development Community (SADC) guidelines call into serious questions the credibility of the elections,” British Foreign Secretary William Hague said.
He urged a thorough investigation of all the violation allegations.
The European Union had early on said it was "concerned about alleged irregularities and reports of the incomplete participation, as well as the identified weakness in the electoral process and a lack of transparency."
SADC's Election Observation Mission (SEOM) had earlier described the elections as "free and peaceful."
Mugabe, who will turn 90 next February, making him Africa's – if not the world's – oldest leader, has been at the helm of power in this southern African country since it gained independence from Britain in 1980.
Zimbabwe has been under sanctions by the European Union and the US for the past 13 years following the violent farm invasions of 2000.