The European Union urged Myanmar on Monday to release "as quickly as possible" two Reuters reporters arrested last week.
"The recent arrest in Yangon of two journalists working for Reuters News Agency is a cause of real concern," a spokeswoman for the EU's foreign affairs chief, Federica Mogherini, said.
Journalists Wa Lone, 31, and Kyaw Soe Oo, 27, were arrested last Tuesday evening after they were invited to dine with police officers on the outskirts of Yangon, Myanmar's largest city.
Little is known of the allegations against the two journalists, other than that police said they were "arrested for possessing important and secret government documents related to Rakhine State and security forces."
"We expect the Myanmar authorities to ensure the full protection of their rights and to release the journalists as quickly as possible," Mogherini's spokeswoman said.
"Freedom of the press and media is the foundation and a cornerstone of any democracy."
The President of the European Parliament, Antonio Tajani, had expressed similar concerns on Friday at an EU summit in Brussels.
Earlier on Monday, Myanmar's civilian President Htin Kyaw authorised police to proceed with a case against the two detained reporters accused of violating the country's colonial-era Official Secrets Act.
Mogherini's spokeswoman said the EU was aware that the arrests may be connected to the journalists' reporting on the crackdown in Rakhine on the Rohingya Muslim minority. The two had reported on the issue in the past.
"The EU reiterates its call on the Myanmar government to ensure transparency and to make serious efforts in order to hold accountable all those involved in serious human rights abuses in the context of the ongoing crisis in Rakhine State," the spokeswoman added.
The EU has sanctions in place against Myanmar barring the sale of arms and equipment that can be used for repression. In response to the Rohingya crisis, in October it also suspended invitations to Myanmar military officers.