WASHINGTON (AFP) - The United States on Friday condemned tough jail terms handed down on five Myanmar journalists for publishing a story about an alleged chemical weapons factory.

State Department spokesman Jen Psaki said Washington was concerned about the jail terms of 10 years with hard labor given to reporters from the Unity Weekly News under the former junta-run country's official secrets act.

"The sentence sends the wrong message about Burma's commitment to freedom of expression, including for the press," Ms Psaki said, using Myanmar's former name.

"The Burmese government has made tremendous progress in the last three years, working to develop an environment conducive to free, fair, independent media," she told reporters.

"This is a critical element of a vibrant and well-functioning democracy and we urge the government of Burma to continue that trend and respect the right of all journalists."

The jailed men, who include the paper's chief executive, were arrested in February just days after the article alleging that the country's military was operating a chemical weapons factory in the town of Pauk in the central Magway region, under the instructions of former junta chief Than Shwe.

Unity Weekly quoted testimony from local people and workers, and included pictures of the alleged military facility.

In January 2013, Myanmar denied accusations it had used chemical weapons against ethnic minority rebels in the northern state of Kachin, with government spokesman Ye Htut saying the military "never" deployed them.