The November 28 polls, whose outcome has already been rejected by the opposition, were seen as crucial to reinforcing stability but have been marred by poor organisation, delays, violence and accusations of widespread fraud.
The United States has closely followed observer missions including the U.S.-based Carter Center which last week said the results lacked credibility, Ambassador James Entwistle said.
"The United States believes that the management and technical execution of these elections were seriously flawed," Entwistle said in an emailed statement to Reuters.
"(They) lacked transparency and did not measure up to the positive democratic gains we have seen in recent African elections."
The elections are the first Congolese-organised polls since the end of a devastating war in 2003 which left millions dead. An earlier poll in 2006 was organised under the auspices of the United Nations.
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