The government of Cambodia has shut down two independent radio stations and ordered the foreign staff of an American NGO to leave the country in the latest move against critics ahead of a general election next year.

The orders on Wednesday came a day after long-time Prime Minister Hun Sen threatened the Cambodia Daily, one of the country's few remaining critical newspapers, with closure over an alleged unpaid tax bill of more than $6m, calling them "thieves".

Maha Nokor and Voice of Democracy, two Khmer-language radio stations, were ordered to close on Wednesday by the ministry of information.

Maha Nokor, which leases programme time to US-funded broadcasters, is one of the few outlets for the opposition to express their views.

The station said it received a letter from Information Minister Khieu Kanharith cancelling its authorisation to operate.

WATCH: Cambodia's Deadly Politics

Yim Sovann, a spokesman for the Cambodia National Rescue Party, the station's main backer, confirmed the government's order.

"I cannot guarantee whether or not the upcoming general election in 2018 will be free and fair," he said.

The manager of Voice of Democracy, primarily an online radio station that bought time on a separate broadcasting station, also said that the station's owner told him he could no longer lease time because of technical and administrative issues.

The manager, Pa Nguon Tieng, said the Voice of Democracy was given seed money by the US government and has since received funds from the European Union and the Danish and Swedish governments.

The closures were announced on the same day the Cambodian foreign ministry ordered the foreign staff of the National Democratic Institute (NDI) to leave the country within a week and halt operations.

The government has accused the institute of violating laws on non-governmental organisations and taxes. It receives funding from the US Democratic Party and promotes democracy and election monitoring worldwide.

'Wider crackdown'

The move against NDI was foreshadowed last week by the appearance of a mysterious Facebook page that accused it of conspiring with the opposition Cambodia National Rescue Party against the government.

Emails and phone calls made by AFP news agency to the institute for comment were not immediately answered.

The government has recently announced a crackdown on organisations it says are delinquent in paying taxes, including foreign and local media and civil society organisations.

On Tuesday, Hun Sen demanded that the English-language The Cambodia Daily pay $6.3m in alleged back taxes and interest by September 4 or face being shut down.

Apart from the American-owned newspaper, the US-funded Radio Free Asia and Voice of America have also been legally targeted. Both stations lease broadcast time from local radio stations.

All have denied wrongdoing and said they are being targeted for their independent reporting.

In a statement, the Overseas Press Club of Cambodia has called for "transparency, fairness and due process" over the threatened closure of The Cambodia Daily.

The organisation said the "targeting" of the paper could be part of a "wider crackdown" on media outlets and civil society ahead of the elections.

Analysts said the cascade of legal cases is straight from the political playbook of Prime Minister Hun Sen, who has cornered opponents throughout his three-decade rule.

Hun Sen's ruling party is expected to face a strong challenge from the opposition in the 2018 elections.