The United States on Thursday urged North Korea to free an imprisoned American on "humanitarian grounds" and not to politicize his case.

North Korea threatened harsher punishment for Aijalon Gomes, sentenced to eight years of hard labor for an illegal border crossing in January, unless Washington drops a campaign to censure it for the sinking of a South Korean warship.

"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.

The ambassador of Sweden, which represents US interests in Pyongyang in the absence of diplomatic relations, was able to see Gomes last on June 10, the seventh such visit, Toner said.

The Korean Central News Agency (KCNA) said 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.

"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.

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

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