The remarks came a few days after Hun Sen called on Cambodians to learn Mandarin Chinese as part of a wider pivot towards Beijing.
Radio Free Asia last week announced the closure of its news bureau in Cambodia after 20 years of operation.
Less known than Hmong and Montagnard fighters, the ethnic Cambodians have congressional, veteran allies who want their efforts recognized.
More than 1,600 Americans remain unaccounted for as a result of the Vietnam war, with about 48 thought to be missing in Cambodia.
Diplomatic efforts have so far failed to halt the county's descent into authoritarianism ahead of elections next year.
In August, CNN reported that President Donald Trump’s administration would impose visa sanctions on four countries, including Cambodia, over their refusal to accept deportees.
Radio Free Asia closes local operation as anti-US rhetoric intensifies.
Son Chhay, the opposition party’s chief whip, said the party believed that international pressure on Phnom Penh would lead to an easing of tensions.
Ambassador William Heidt also calls for immediate release of opposition politician Kem Sokha, saying his arrest threatens country's fragile democracy.
The Daily closed on September 4 after more than 24 years amid a standoff with the country’s tax agency, which handed it a $6.3 million tax bill last month.
Hun Sen has recently said he will rule Cambodia for a further decade, bringing his reign to over 40 years.
Arrest of party leader, threat from Prime Minister Hun Sen fails to discourage Cambodia National Rescue Party from challenging autocratic leader.