The US State Department says it is committed to working with the new president of the Democratic Republic of Congo, Felix Tshisekedi, who is to be sworn in on Thursday.
But it has urged him to address reports of electoral irregularities.
The US had previously condemned the lack of transparency over the result, which many observers say was rigged by Mr Tshisekedi and the outgoing president, Joseph Kabila.
A rival opposition candidate, Martin Fayulu, says he won about 60% of the vote, but his challenge was dismissed by the Constitutional Court.
Following the Constitutional Court ruling African presidents also came out to congratulate Mr Tshisekedi.