But State Department spokesman Sean McCormack said the United States would withhold judgment on the results of Sunday's vote -- which Macedonia's ruling conservatives apparently won -- until the final results were in.
"Quite clearly there were some very serious problems," McCormack told a news briefing.
"We would call upon the government of Macedonia to rerun elections in those districts where people were not able to cast their ballot free from threat of violence or intimidation, (and) to prosecute those responsible."
However, McCormack said Macedonia's bid to join the NATO alliance should still be able to go ahead, once the election problems were remedied and a dispute with Greece over the country's name was solved.
Election monitors criticized a failure to prevent violence in ethnic Albanian areas of Macedonia that killed one man and wounded nine others on Sunday. The monitors also noted instances of intimidation and ballot-box stuffing.
In rerunning the elections, Macedonia's government should "ensure that proper security forces are in place so that an election can take place in an environment where people are able to vote," McCormack said.
He said U.S. officials had spoken with Macedonia's government about the matter, and were confident Macedonia would move "quickly and effectively" to fix the problem.
The reported results would give the VMRO-DPMNE party of Prime Minister Nikola Gruevski the healthiest majority in parliament in more than a decade, riding a wave of nationalist anger over Greece blocking Macedonia's NATO membership invitation in April.
Greece blocked Macedonia's bid to join the alliance because of long-standing dispute over the Balkan country's name, which is the same as that of Greece's northern province. Earlier this month, U.S. Secretary of State Condoleezza Rice pledged continued strong support for Macedonia's membership bid.
"I would expect that once the name issue is resolved, in the context of NATO, that their membership should be able to move forward," McCormack said, adding: " Of course, we would expect that they would remediate, effectively and quickly, this current situation."