In a statement, the embassy said the Haitian government and the country’s provisional electoral commission needed to explain why a number of candidates won seats in the final results, when they hadn’t been leading in preliminary counts.
Of 18 such cases, it said, 16 favoured the governing Unity party – known in Haiti as Inite – of the outgoing President, Rene Preval.
Haiti’s President-elect, Michel Martelly, has called for an investigation into the results and urged Mr Preval not to ratify them.
Mr Martelly, a popular Haitian singer, won the presidency in a run-off election, with over two-thirds of the vote. But his party has won only a handful of seats in the incoming legislature.
So if he is to get much-needed legislation passed, he will have to work alongside the Unity party, which will dominate the new legislature.
It will have an absolute majority in the 30-seat Senate. And it is very close to an absolute majority in the lower house, which has 99 seats