A Russian court has ruled to keep a Norwegian man in jail pending a probe on charges of espionage that he has rejected.

Frude Berg has been in custody since his December's arrest in Moscow. Moscow's Lefortovsky district court ordered Friday to extend his arrest through May.

The Russian Federal Security Service, or FSB, the main KGB successor agency, has accused Berg of being part of a spying operation.

Russian news reports quoted Berg's lawyer Ilya Novikov as saying that Berg has denied the charges. Novikov said the court rejected his petition to free Berg on bail. He said he would appeal the ruling.