Reuters news agency called on Myanmar to immediately release two of its journalists who were arrested for possessing "important secret papers" obtained from two policemen who had worked in Rakhine state, where violence widely blamed on security forces has forced more than 630,000 minority Rohingya Muslims to flee into neighbouring Bangladesh.

The Ministry of Information said Wednesday the journalists and policemen will be charged under the country's colonial-era Official Secrets Act, which carries penalties of up to 14 years in prison.

Reuters said Wa Lone and Kyaw Soe Oo had been missing since late Tuesday night.

Read more: Two Reuters reporters arrested in Myanmar for possession of 'important security documents': govt

"Reuters reporters Wa Lone and Kyaw Soe Oo have been reporting on events of global importance in Myanmar, and we learned today that they have been arrested in connection with their work," Stephen J. Adler, president and editor In chief of Reuters, said in a statement.

"We are outraged by this blatant attack on press freedom. We call for authorities to release them immediately," he said.

The ministry posted a photo of the two journalists in handcuffs, standing behind a table bearing documents, cellphones and currency.

It said they had collected "information and important secret papers related to the security forces" from the policemen, who had earlier worked in Rakhine but were now in Yangon, the country's largest city.

"When we saw that photo of them, it broke our hearts that their picture was taken like they're criminals," said War Lay, a sister of Kyaw Soe Oo. "He was just doing his job as a journalist and we hope that they will be released soon."

Rakhine state is the epicentre of the Myanmar military's brutal security operation against Rohingya Muslims.

The campaign, launched in August in response to attacks on police outposts, has been condemned by the United Nations as "ethnic cleansing" and those fleeing have described widespread rights abuses by security forces.

In Tokyo, United Nations Secretary-General Antonio Guterres expressed concern over the detention of the journalists, saying it was part of an erosion of press freedom in Myanmar.

"I think it's important that the international community does everything possible" to win the release of the journalists and also to end the dramatic human rights violations that have caused many to flee their country, he said Thursday.

The military, which is charge of security in northern Rakhine, and the civilian government have barred most journalists and international observers from independently traveling to the region.

Shawn Crispin, a senior representative for the Committee to Protect Journalists, also called on Myanmar authorities to "to immediately, unconditionally release" the journalists.

"These arrests come amid a widening crackdown which is having a grave impact on the ability of journalists to cover a story of vital global importance," he said.

The Foreign Correspondents Club of Myanmar said it was "deeply shocked" by the arrests and "gravely concerned with the state of press freedom in Myanmar as the journalists were detained while carrying out their journalistic work."

It also called on authorities to allow their families to meet them as soon as possible.

The US Embassy said it was "deeply concerned by the highly irregular arrests of two Reuters reporters."

"For a democracy to succeed, journalists need to be able to do their jobs freely," it said in a statement.

"We urge the government to explain these arrests and allow immediate access to the journalists."

Journalists in Myanmar are facing renewed harassment, with several arrested in recent months.

Two foreign journalists along with two of their Myanmar associates are currently awaiting trial on new charges after already being sentenced to jail for illegally flying a drone over parliament.