North Korea rejected U.S. calls on Thursday for the release of an American sentenced to eight years of hard labour, saying it was considering how to make the punishment even harsher under rules of war.
Aijalon Mahli Gomes, 30, from Boston, was convicted in April of illegally entering the country, a judgment that has further strained ties with the United States as Washington backs Seoul in seeking U.N. censure for North Korea's suspected sinking of a South Korean navy ship.
Impoverished North Korea, which denies any involvement in the sinking, has in the past used detained American citizens as bargaining chips.
"The U.S. government has requested the release of Gomes out of humanitarian concern, but not only is this impossible under the current situation but the question is how to increase the punishment," the North's official KCNA news agency said.
KCNA accused Washington of hostility, saying Washington's actions in the wake of the South Korean navy ship sinking in effect meant "a state of war".
"Related agencies are reviewing what additional measures can be taken under the laws applicable in times of war against the U.S. citizen Gomes who is now in a correctional system under the grave crime of infringing on the republic's sovereignty."
Gomes had been teaching English in Seoul for about two years before making the trip to North Korea in January. He was also active in Protestant churches, his colleagues said.
He likely crossed into North Korea in support of U.S. Christian missionary Robert Park who entered the North on Christmas Day to raise awareness about its human rights abuses, according to an activist who helped arrange Park's trip. Park was detained and eventually released.
In previous cases, North Korea, the focus of multinational talks aimed at reining in its nuclear weapons programme, has typically released Americans a few months after their capture after trying to win concessions.
In a separate incident, two Chinese nationals died while being interrogated by North Korean authorities on charges of espionage, South Korea's Yonhap news agency said, quoting Chinese sources.
The pair, who entered the North as merchants, were severely beaten after their arrests, Yonhap said. The North has refused to release their bodies, fuelling diplomatic friction between the two states, it said.
North Korea has said it would punish those responsible for the deadly shooting of three Chinese nationals in a separate incident near their border early this month and vowed to prevent any repeat.