The Obama administration has told North Korea not to link the case of a detained American Christian with the efforts to censure the country for the alleged sinking of a South Korean warship.

The U.S State Department said Thursday that North Korea should "separate political rhetoric from this matter concerning a private American citizen." The request came after North Korea cited hostile U.S. policy and threatened to increase punishment for Aijalon Mahli Gomes.

"The U.S. government is requesting the (North) to leniently set him free from a humanitarian stand, but such thing can never happen under the prevailing situation and there remains only the issue of what harsher punishment will be meted out to him," Korean Central News Agency said.

North Korea has freed three other Americans held for illegal entry into the communist country, but won't release Gomes amid tensions over the sinking of the South Korean warship that Seoul and Washington have blamed on the North.

A North Korean court sentenced him in April to eight years of hard labor and fined him $700,000 for entering the country illegally.

Gomes attended rallies in Seoul in support of a fellow Christian who had crossed into North Korea to protest its human rights record.