The United Nations have urged Egypt to release detained foreign journalists, including an Australian, accused of airing false news.

The United Nations High Commissioner for Human Rights has called for the release of foreign journalists, including Australian Peter Greste, detained in Egypt.

Mr Greste, 48, and two al-Jazeera English network colleagues were arrested at a Cairo hotel on December 29 on suspicion of broadcasting false news in the service of the black-listed Muslim Brotherhood.

Egyptian prosecutors have since referred to trial 20 al-Jazeera journalists, including Mr Greste, for airing false news.

UN High Commissioner for Human Rights spokesman Rupert Colville says the move, based on “vague charges”, was of great concern.

In a statement delivered in Geneva on Friday night, Mr Colville said the prosecutors’ announcement had increased fears among the general media.

“We urge the Egyptian authorities to promptly release all journalists imprisoned for carrying out legitimate news reporting activities in exercise of their fundamental human rights.”

Australian Foreign Minister Julie Bishop has spoken with her Egyptian counterpart Nabil Fahmy to express her concerns about the charges, but warns there is only so much the Australian government can do.

“Clearly, it is not possible for another nation to interfere in the criminal proceedings of another country,” she told ABC Radio on Friday.

Ms Bishop said Mr Fahmy had offered some assurances about Mr Greste’s situation.

“He did give us an assurance that as far as he was concerned, the case would be dealt with expeditiously and fairly,” she said.

“We will continue to press our position that he should be given conditional release while we consider what more we can do to seek his release overall.”

Australia’s ambassador to Egypt Ralph King on Thursday night was briefed by the country’s prosecutor-general about the nature of the charges facing the journalist.

That information was passed on to Mr Greste’s Brisbane-based parents, who have described their son’s detention as the most harrowing incident of their life.

From his solitary confinement Greste penned a letter to his parents.

“I’ve been caught in the middle of a political struggle that is not my own,” the reporter wrote.

“The state will not tolerate hearing from the Muslim Brotherhood or any other critical voices.”

Greste, usually East Africa correspondent for the global network, was due to work in Egypt for only a short time, relieving a colleague.