Political tensions have been rising between Phnom Penh and Washington, after Cambodia ordered a USAID-supported organization to halt its activity in the country. The Asian nation says it is defending its sovereignty, while Washington questioned its democracy.

In the latest development to the worsening dispute between the two nations, Cambodia's long-ruling leader, Prime Minister Hun Sen, accused Washington of interference in his country's domestic affairs.

"We wish to send a clear message again to the US Embassy that we defend our national sovereignty," the Cambodian government said in an open letter Thursday, as quoted by Reuters.

"Cambodians are well aware of what a democratic process means. You do not need to tell us what it is," the letter added, while also describing American democracy as "bloody and brutal."

Phnom Penh also reportedly blamed Washington for contributing to the rise of the Khmer Rouge regime in the 1970s. Formed as an offshoot of forces in neighboring Vietnam during the Vietnam War, in which the US was involved, the rule of the Red Khmers was marked by the genocide of nearly 2 million Cambodians.

Earlier this week, the Cambodian Foreign Ministry announced it was expelling the US-funded National Democratic Institute (NDI) after a series of publications in local media suggesting that the non-profit organization attempted to act against the government. Phnom Penh said the NGO had violated national registration rules.

Breaking: Foreign Ministry order @NDI to stop all of its operation in #Cambodia, and their foreign staffs shall leave the country in a week. pic.twitter.com/vGGgaI3RIE — Chhengpor Aun (@aunchhengpor) August 23, 2017

Although in its mission statement the institute, which has been operating in Cambodia since 1992, claims that it works with all political forces, some of its materials leaked online allegedly showed its political bias and moves to assist the opposition to overthrow the government.

The intentions of other NGOs operating in Cambodia and funded by USAID (US Agency for International Development) have also been questioned by the media, suggesting those organizations might be tools of the US State Department promoting Washington's foreign policy.

"There are... NGOs who also will want to create a color revolution and topple the government like they have successfully in other countries," one of the publications on Cambodia's Fresh News said.

Saying that it's "surprised and saddened by this development," the US institute claimed it has been "transparent in our work, and have made every attempt to comply with the law." The NDI said in a statement that it "has worked with all major political parties, including the ruling party," adding that it hoped the government would review its decision.

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The US Embassy in Phnom Penh has harshly criticized the Cambodian government, having questioned its commitment to democracy. The US State Department followed suit, saying it "is deeply concerned by the deterioration in Cambodia's democratic climate."

Having called the Cambodian government’s actions "troubling," Washington also accused Phnom Penh of hampering the work of several "opposition-aligned" and "independent media outlets" in the country.

Earlier this year, the Cambodian government released an official document, stating that the "United States was funding overseas activities with the aim to change a regime the US administration doesn't like."

Citing the examples of Ukraine and Honduras, the government said that "Cambodia has been facing an increasing campaign led by some foreign governments and organizations that want a regime change in Cambodia."

"This campaign, fueled by distortion of facts, lies, amplification of minor problems, aims to discredit and blame the existing and legitimate institutions," it said, adding that some of over 6,200 registered associations and NGOs in the country have been "deeply engaged in political activities funded by foreign institutions and governments."