The CPP swept Sunday's general election, but there were doubts over the victory's legitimacy after the country's highest court outlawed and dissolved the main opposition Cambodia National Rescue Party last year, allowing Hun Sen to essentially run unopposed, Efe reported.

"The flawed elections, which excluded the country's principal opposition party, represent the most significant setback yet to the democratic system enshrined in Cambodia's Constitution," White House press secretary Sarah Huckabee-Sanders said in a statement.

Sunday's elections "substantially erode Cambodia's achievements in promoting political reconciliation and economic growth since the 1991 Paris Peace Agreement", the statement read.

The White House added that it was "profoundly disappointed in the government's choice to disenfranchise millions of voters", pointing out that "genuine democracies tolerate opposing political views, foster competition through elections, and promote and protect the free exchange of ideas".

"In contrast, in the months leading up to the vote, the Cambodian government placed ever tighter restrictions on independent media and civil society, dissolved the main opposition party, jailed the opposition leader," Huckabee said, before concluding with a warning: "The United States will consider additional steps to respond to the elections."

Around 8.3 million Cambodians were registered to vote in Sunday's elections, in which 19 other candidates, mostly from small, recently formed parties, were running against the CPP and Prime Minister Hun Sen, who has ruled the southeast Asian nation since 1985.