The White House Wednesday called on Malaysia to address "concerns" about an election which drew opposition claims of fraud but nevertheless congratulated the governing party's winning candidate.
"We note concerns regarding reported irregularities in the conduct of the election, and believe it is important that Malaysian authorities address concerns that have been raised," said White House spokesman Jay Carney.
"We look forward to the outcome of their investigations."
On behalf of President Barack Obama, Carney also congratulated Prime Minister Najib Razak's Barisan Nasional (National Front) coalition, which has ruled the country for the last 56 years.
He also noted the record numbers of Malaysians who turned out to vote in the parliamentary polls on Sunday, and opposing parties, as "vibrant opposition is a foundation of democracy."
The White House statement was released as thousands of Malaysians dressed in mourning black gathered to denounce the elections, which they say were stolen through fraud.
Opposition leader Anwar Ibrahim has pledged to produce evidence from the closest-ever Malaysian polls that he says will show the government is "illegitimate."
The US State Department had on Monday congratulated Barisan for its win but noted "irregularities" in the voting.
But it called on all parties to "peacefully respect the will of the voters."
As part of a new focus on Southeast Asia, the Obama administration has reached out to Najib to improve relations with Malaysia, which were uneasy during the 1981-2003 rule of firebrand Mahathir Mohamad.
Anwar, who spent six years in prison on charges he said were trumped up by Mahathir over a power struggle, has also been a frequent visitor to Washington as he tried to cast himself as a voice of democracy and moderation.