WASHINGTON (AP) — The U.S. on Monday repeated its concerns about irregularities in Cambodia's elections but did not criticize authorities for ratifying the results without addressing them.

Cambodia's state election board ratified Sunday the victory of the party of Prime Minister Hun Sen, who has ruled for 28 years.

The opposition says it will boycott the opening session of parliament and stage protests next week unless there's an independent review of reported irregularities in the July 28 election which it claims robbed it of votes.

In Washington, State Department spokeswoman Marie Harf said the U.S. still believes a transparent review of irregularities would give Cambodians greater confidence in their electoral system.

But she wouldn't be drawn on the results' ratification.

"I'm not going to say it's a good or a bad thing. We still have concerns about some irregularities," Harf told reporters.

U.S. lawmakers have previously called for cuts in the more than $70 million in annual aid to Cambodia's if the election is not free and fair.

Fears of political violence in Cambodia could weigh upon U.S. policymakers as they mull their response. The opposition, led by Sam Rainsy, won significantly more seats than it had in the past. Mass protests risk drawing a tough response from Cambodian security forces under Hun Sen's control.

Rainsy told reporters his party would not take part in any form of government until there's been an independent probe. But there's speculation the opposition could be seeking a show of strength to crank up pressure on the ruling party to give up some of the leadership positions in the next government.

Human Rights Watch said Monday the U.S. should not recognize the elections as free and fair.

"The facts are clear: The Cambodian election failed as a vehicle for representing the democratic will of the Cambodian people," said John Sifton, the group's advocacy director for Asia.