A U.S. expert on North Korea says that over one million North Koreans may be listening to messages broadcast to the North from the U.S. and South Korea and that the number continues to increase.

Peter Beck, a research fellow at Stanford University's Asia Pacific Research Center, made the remark in an essay run by the Wall Street Journal.

Beck said that despite North Korean authorities supplying only fixed-frequency radios blocking overseas broadcasts, North Koreans can still listen to foreign broadcasts by tampering with the radios or by listening to radios smuggled in from China.

He cited North Korean defectors as saying that foreign radio-listening is one of the top motivations to defect.

Voice of America and Radio Free Asia out of the U.S. and more recently three radio stations operated by North Korean defectors, including Free North Korea Radio, are broadcasting to the communist state.