MOSCOW (Reuters) - U.S. troops are in the conflict zone in eastern Ukraine and are training Ukrainian forces, Interfax news agency quoted a spokesman for Russia's Defense Ministry as saying on Thursday.

A day earlier, Washington accused Russia of building up air defense systems inside eastern Ukraine and of involvement in training exercises of pro-Russian rebels in breach of a European-brokered truce.

Igor Konashenkov was quoted as saying that U.S. troops were training Ukrainian forces not only in western Ukraine "as Ukrainian TV channels show, but directly in the combat zone in the area of Mariupol, Severodonetsk, Artyomovsk and Volnovakha".

The Defense Ministry confirmed the report.

(Reporting by Gabriela Baczynska, editing by Elizabeth Piper)