A Cambodian court extended the pre-trial detention of two Radio Free Asia journalists who've been in prison for six months while espionage charges are investigated.

The journalists, Uon Chhin and Yeang Socheamet, were arrested in November and charged with undermining Cambodian national security by supplying information to a foreign state.

The charge, which both men deny, has a maximum penalty of 15 years' imprisonment.

Their lawyer said the law allows for them to be released if they haven't been tried within six months.

But a spokesman for Phnom Penh Municipal Court, Ei Rin, said a judge extended their detention because the investigation isn't completed.

The increasingly authoritarian government of Prime Minister Hun Sen has shut down more than a dozen radio stations and dissolved the opposition Cambodia National Rescue Party.