A diplomatic row has broken out between Poland and the US after the American ambassador in Warsaw accused senior Polish officials of attempting to intimidate journalists from a US-owned broadcaster.

The dispute concerns the ongoing controversy surrounding an explosive documentary that was broadcast in January on TVN24, the news station of private broadcaster TVN, in which reporters infiltrated a Polish neo-Nazi organisation and broadcast footage of its members holding a birthday party for Adolf Hitler in a Polish forest – complete with a birthday cake decorated with the colours of the Third Reich.

The footage fed concerns over the rise in visibility of Polish far-right movements in recent years, with critics accusing Poland’s ruling rightwing Law and Justice party (PiS) of turning a blind eye to radical nationalist sentiment.

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Senior Polish officials, including the interior minister, Joachim Brudziński, have appeared to endorse recent allegations in pro-government media outlets – based on the testimony by one of the arrested neo-Nazis – that the event in the documentary had been staged by the journalists as an anti-government provocation.

That prompted the US ambassador, Georgette Mosbacher, a Republican donor and Donald Trump nominee who took up her post in September, to write a letter to the Polish prime minister, Mateusz Morawiecki, about her “deep concern” over the allegations.

“It is astonishing that these public figures would attack journalists who were fulfilling the functions of an independent media in Poland’s vibrant democracy,” Mosbacher wrote.

“I understand that following the initial broadcast [in January], Polish law-enforcement authorities launched actions against members of the extremist group that organised the event. It is baffling that members of the government now appear more interested in casting doubt on the motives of the journalists and attempting to associate TVN with extremism.”

In a bizarre twist, it emerged this week that Polish prosecutors, who operate under the direct oversight of the justice ministry, had initiated a criminal investigation into the TVN cameraman who had infiltrated the neo-Nazi organisation to film the ceremony, on suspicion of charges of propagating fascism. According to TVN, agents from Poland’s Internal Security Agency visited the cameraman at his home last week.

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Mosbacher’s letter, which misspelt the names of both Morawiecki and Brudziński and was sent earlier this month, but leaked this week to the Polish press, has prompted outrage in Polish government circles. In a series of tweets on Monday evening, ruling party MP Krystyna Pawłowicz accused Mosbacher of “supporting TVN’s anti-Polish activity”, alleging that it was engaged in a plot to overthrow the government. “I DEMAND [from Mosbacher] respect for the Polish state and nation, and its democratically elected authorities,” she wrote.

A spokeswoman for the Polish government said in a statement that Warsaw and Washington enjoy “very good relations and one incident will not change that”.

This is the second time in less than a year that the American and Polish authorities have clashed over TVN, Poland’s leading private broadcaster, which is owned by the Tennessee-based Scripps Networks Interactive.

In December 2017 the Polish government was caught aback by the strength of the American response after Poland’s broadcasting regulator, which is controlled by Law and Justice, issued a fine against TVN for its coverage of anti-government protests in 2016 on the basis of a report that seemingly implied that the broadcaster was being punished for failing to explicitly condemn the demonstrations.