LONDON (Reuters) - A U.S. military serviceman detained in Russia on suspicion of theft has also been charged with threatening to kill his girlfriend, Russian independent outlet Mediazona reported on Monday, citing the local prosecutor's office.

Prosecutors in Vladivostok in Russia's Far East said they had approved an indictment of a U.S. citizen born in 1989. It did not name the U.S. citizen but the details matched Black's.

The prosecutor's statement said the two had quarrelled and the American had subsequently "forcibly grabbed the girl by the neck, which she perceived as a real threat to her life". It did not say how or whether the accused had responded to the new charge.

Black, a 34-year-old U.S. staff sergeant, was detained on May 2 by police in Vladivostok on suspicion of stealing from a Russian woman he was in a relationship with.

He has pleaded guilty to the theft charges and is cooperating with investigators. A Russian court ruled last week to keep him in pre-trial detention.

The United States warns its citizens against all travel to Russia, citing an array of reasons, including "the potential for harassment and the singling out of U.S. citizens for detention by Russian government security officials" and the arbitrary enforcement of the law.

(Reporting by Reuters; Writing by Lucy Papachristou, Editing by Timothy Heritage)