The European Union's envoy to Myanmar has raised concerns about the arrest of two Reuters journalists in a letter to the country's leader, Aung San Suu Kyi.

Kristian Schmidt described the situation as "serious intimidation" and called for their immediate release.

The Reuters journalists, Wa Lone, 31, and Kyaw Soe Oo, 27, were detained on 12 December.

They are being investigated on suspicion of breaching the Official Secrets Act, a little-used law that dates from the days of British colonial rule.

They had worked on coverage of a crisis in the western state of Rakhine, where a military crackdown that followed militant attacks on security forces in August led to an exodus of more than 650,000 Rohingya Muslims to refugee camps in Bangladesh.

The two are due to appear in court tomorrow.

It will be their second appearance in court and the prosecutor could request that charges are filed against them.

"This situation amounts to a serious intimidation against journalists in general and from Reuters in particular," said Mr Schmidt in the letter dated 8 January.

"Journalists should ... be able to work in a free and enabling environment without fear of intimidation or undue arrest or prosecution," he said.

"We therefore call on your government to provide the necessary legal protection for these two journalists, to ensure the full respect of their fundamental rights and to release them immediately."

The journalists were detained after they were invited to meet police for dinner in Yangon.

The Ministry of Information has cited the police as saying they were "arrested for possessing important and secret government documents related to Rakhine State and security forces".

It said the men had "illegally acquired information with the intention to share it with foreign media".

Government officials from some of the world's major nations, including the United States, Britain and Canada, as well as top UN officials, have called for their release.

Reuters President and Editor-In-Chief Stephen J Adler has called for the immediate release of the two.

"As they near their hearing date, it remains entirely clear that they are innocent of any wrongdoing," Mr Adler said in a statement yesterday.

Authorities have blocked access to media seeking to cover the military crackdown in the north of Rakhine State.

The United Nations has condemned the military campaign there as ethnic cleansing, a charge Buddhist-majority Myanmar has rejected.