North Korea has threatened harsher punishment for a jailed US citizen who is serving eight years' hard labour for illegally entering the country.
Aijalon Mahli Gomes, a former English teacher in South Korea, was arrested in January after crossing from China.
North Korea said it would use "wartime law" against the 30-year-old if the US continued its "hostile approach" over the sinking of a South Korean warship.
An international inquiry found a North Korean torpedo sank the Cheonan.
According to North Korea's state news agency, US requests to free Gomes will not be accepted while the dispute over the sinking of the warship continues.
Instead the Korean Central News Agency says "there remains only the issue of what harsher punishment will be meted out to him".
"If the US persists in its hostile approach, the latter will naturally be compelled to consider the issue of applying a wartime law to him," state media reported.
Analysts say "wartime law" could mean a life sentence or the death penalty.
The Cheonan went down near the disputed maritime border in March, with the loss of 46 South Korean crew.
Investigators said they had discovered part of a torpedo on the sea floor and it carried lettering that matched a North Korean design.
Pyongyang rejected the claim as a "fabrication" and threatened war if UN sanctions were imposed.
This latest statement by North Korea raises the stakes in the confrontation still further.
Gomes was the fourth American citizen to be accused of entering the country in the past year.
In February, North Korea freed Robert Park, who had entered the country from China by walking over a frozen river.
Last year two US journalists, Laura Ling and Euna Lee, were also arrested by North Korea on the border with China.
They were sentenced to 12 years' hard labour but freed after four months, as part of a diplomatic mission spearheaded by former US President Bill Clinton.