CNRP Deputy Director of Foreign Affairs Monovithya Kem told a news conference in Jakarta on Monday the party welcomed the White House statement and hoped others would follow suit.
The CNRP also called on the international community to reject the result of the election.
"29 July 2018 marked the death of democracy in Cambodia, a dark new day in recent history," said CNRP vice president Mu Sochua. "The result announced by the CPP and the National Election Committee must be fully rejected by the international community."
Newspaper headlines in Cambodia on Monday greeted Hun Sen's victory.
"CPP Dominates Election: Unofficial results show ruling party will increase its majority in the National Assembly," said the pro-government Khmer Times newspaper.
Some independent media were targeted by Hun Sen and his allies in a pre-election crackdown.
More than 82 percent of those registered to vote cast a ballot, according to the National Election Committee. Turnout was 90 percent in the 2017 local election and 69.61 percent in the previous general election in 2013.
Polling stations visited by Reuters in the capital, Phnom Penh, were quiet on Sunday and some voters said they voted because not doing so would land them in trouble.
The CNRP had called on voters to boycott the election but authorities warned anyone doing so would be seen as a traitor.
At the Kapkor Market in Phnom Penh, Meas Sinuon, 60, said she was pleased with Sunday's result.
"This government has done good things already," she said as she shopped for groceries. "But I want the new government to do more to help the people."
Another voter, a Phnom Penh tuk-tuk driver who declined to be identified, described the result as "same".
Unofficial election results are expected in mid-August.
