Authorities in North Korea continue their silence about the fate of an American missionary who crossed into the country on Friday.

Human rights activists in Seoul said on Saturday that Robert Park, a Korean-American, crossed into North Korea on Christmas Day to call attention to human rights abuses in the reclusive communist state.

The activists said Park crossed into North Korea from China, carrying a letter urging North Korean leader Kim Jong Il to release political prisoners, shut down the concentration camps where they are held, and open borders to allow aid teams to enter the country.

South Korea and Western governments claim North Korea maintains a network of prisons and labor camps holding tens of thousands of political prisoners.  Pyongyang denies the charges.

A colleague of Park said he is a member of an international campaign called "Freedom and Life for All North Koreans."

American journalists Euna Lee and Laura Ling of the U.S. media outlet Current TV, were arrested in March after accidentally crossing into North Korea while working on a story.

They were sentenced to 12 years of hard labor, but were freed as part of a diplomatic mission to Pyongyang in August led by former U.S. President Bill Clinton.

Some information for this report was provided by AP and Reuters.