North Korea Thursday threatened harsher punishment for a US citizen sentenced to hard labour unless Washington drops a campaign to censure it for the sinking of a South Korean warship.

The announcement raised the stakes in the confrontation over the ship, which Seoul and Washington say was broken apart by a North Korean torpedo near the disputed border in March.

Aijalon Gomes is serving eight years of hard labour for an illegal border crossing in January.

The North can never accept US requests to free him under the current situation “and there remains only the issue of what harsher punishment will be meted out to him”, the Korean Central News Agency (KCNA) said.

“If the US persists in its hostile approach toward the (North), the latter will naturally be compelled to consider the issue of applying a wartime law to him,” it said without elaborating.

An analyst said “wartime law” could mean life imprisonment or even a death sentence.

Washington responded by again urging Pyongyang to free Gomes on “humanitarian grounds” and not to politicise his case.

“We urge the North Korean government… to release Mr Gomes on humanitarian grounds,” said Mark Toner, a State Department spokesman.

“We expect the authorities to treat him in a humane manner, consistent with international human rights law, and finally we urge the authorities there to separate political rhetoric from this matter concerning a private American citizen,” Toner said.

South Korea and the United States are pressing the UN Security Council to censure the hardline communist state over the sinking which cost 46 lives.

The North denies involvement and has threatened military reprisals if the council takes action.

Pyongyang accused Washington of “persistently antagonising” it over the warship issue and described the current situation as a “war phase”.

“An institution concerned is now examining the issue of what additional measure it will take against American Gomes in line with a wartime law,” KCNA said.

Gomes, a former English teacher in South Korea and reportedly a devout Christian, was arrested in January — the fourth US citizen in less than a year to be detained for illegal entry.

The 30-year-old from Boston was sentenced in April to jail and a fine of 70 million won, equivalent to 700,000 dollars at the official exchange rate.

“This is a message to Washington that relations between Pyongyang and Washington could get worse unless the United States eases its pressure over the sinking of the warship,” said Yang Moo-Jin of Seoul’s University of North Korean Studies.

“Under a wartime law, a heavier penalty such as life imprisonment or death is always possible to people from an enemy country,” he told AFP.

Kim Yong-Hyun, a professor at Seoul’s Dongguk University, said it was too soon to say whether the US citizen would be used as a hostage “but North Korea is indicating it can do so if relations are strained further”.

The threat to Gomes came on the eve of the 60th anniversary of the start of the Korean War and coincided with a demand from Pyongyang for 65 trillion dollars in compensation from the United States for six decades of hostility.

Gomes crossed into the North one month after US missionary Robert Park walked over a frozen border river on Christmas Day. Park called on leader Kim Jong-Il to quit because of rights abuses.

A Seoul activist has said Gomes may have been inspired by Park’s example.

North Korea freed Park in February without putting him on trial.

American journalists Laura Ling and Euna Lee, who also crossed the border illegally, were held for five months before being released last August during a visit to North Korea by former president Bill Clinton.

South Korea said Thursday it is still seeking a Security Council resolution over the sinking.

But Foreign Minister Yu Myung-Hwan noted that China and Russia, two of the five veto-wielding permanent council members, have expressed negative opinions about the UN taking “very strong measures”.

“We are pushing for a resolution as a first option… that’s an opinion we share with key UN Security Council members,” Yu told reporters.

But he hinted Seoul might settle for a statement from the council’s president, which he said would be more significant politically. It needs unanimity among members even if it has no binding force.