(Reuters) - An American journalist detained in military-ruled Myanmar accused of incitement is facing new charges of sedition and terrorism, his lawyer said on Wednesday, in a setback for U.S. efforts to secure his release.

       Danny Fenster, 37, who was managing editor of Frontier Myanmar, a top independent news site, was detained at Yangon's international airport in May as he attempted to take a flight out of the country.

       It was not immediately clear what Fenster was accused of in regard to the new charges, which are the most serious levelled against him.

       If found guilty, he could be jailed for up to 20 years under a terrorism law and 20 years for sedition.

       "We don't understand why they added more charges but it is definitely not good that they are adding charges," his lawyer, Than Zaw Aung, told Reuters.

       "Danny also felt disappointed and sad regarding these new charges."

       The United States has repeatedly pushed for the release of Fenster, who was initially charged with incitement and breaches of a colonial-era unlawful associations act. He is being held at Yangon's notorious Insein prison.

       Authorities overlooked him in a recent amnesty for hundreds of people detained over anti-junta protests, which included some media personnel.

       The military has rescinded media licenses, imposed curbs on the internet and satellite broadcasts and arrested dozens of journalists since its Feb. 1 coup, in what human rights groups have called an assault on the truth.

       "We are as heartbroken about these charges as we have been about the other charges brought against Danny," his brother, Bryan Fenster, said in a text message.

       A spokesperson for the ruling military council did not answer calls seeking comment.

       The U.S. embassy in Yangon did not immediately respond to a request for comment.

       (Reporting by Reuters Staff; Writing by Martin Petty; Editing by Robert Birsel)