Eddie Jun Yong-Su, an ethnic Korean US citizen, had been engaged in "aggressive" missionary activities in the hardline communist state, said Lim Chang-Ho, a professor at a South Korean theological college.
Lim said Jun and two ethnic Koreans with Chinese passports were arrested at the same time last November. "The two others were badly beaten but they were allowed to return home as they were Chinese nationals," Lim said.
"According to them, Jun was beaten up so severely that he could hardly walk without help," he said.
It was not possible to confirm the alleged mistreatment.
The North announced last month that Jun would be charged with unspecified crimes against the nation.
Carter appeals for release
The Swedish embassy, which represents US interests in Pyongyang, has been given access to him, and the US State Department said this month the detainee had been allowed to speak to his family by phone.
Former US president Jimmy Carter appealed for Jun's release when he visited the North last month. But the nominal head of state Kim Yong-Nam told Carter the detainee would not be freed.
Jun is an agricultural machinery salesman who attends a church in California's Orange County and makes frequent trips to the North. His family pleaded last month for his release, saying he was in uncertain health after several months' detention and may not survive a trial.
Lim said Jun belongs to a South Korean missionary group which has been supporting underground Christian churches in the North with cash donations and publications.
He said North Korean authorities had warned Jun about his activities but he continued with them.
"He was arrested on the spot as he was establishing contacts with underground Christians," said Lim, who teaches at Koshin University in the southern city of Busan.
"North Korea apparently considers this case as a chance to track down and eradicate underground churches at one blow," he said, adding Pyongyang regards such Christians as a threat to its regime.
Lim said underground churches in North Korea have about 10 000 followers while 30 000 other Christians are behind bars.
The North's constitution guarantees freedom of religious belief. In practice, "genuine religious freedom did not exist", according to the US State Department's latest human rights report.