SEOUL, South Korea - North Korea threatened Thursday to increase punishment for an American who was sentenced to hard labor for illegally entering the country, citing what it called a hostile U.S. policy toward it.

Aijalon Mahli Gomes, from Boston, was sentenced in April to eight years of hard labor and fined US$700,000 for entering the country illegally and for an unspecified "hostile act."

The U.S. has asked North Korea to release him on humanitarian grounds.

Thaleia Schlesinger, a spokeswoman for Gomes' family in Boston, said they had not heard the news and did not immediately have a comment.

Communist North Korea has freed three other Americans detained for illegal entry, but ruled out Gomes' release amid tensions over the sinking of a South Korean warship that Seoul and Washington have blamed on the North.

South Korea has asked the UN Security Council to censure North Korea over the sinking. The North denies it was responsible and has warned that any moves to punish it at the UN could lead to armed conflict and possibly nuclear war.

The U.S. and South Korea have urged North Korea to avoid provocations and vowed to hold it accountable for the sinking of the warship in March, in which 46 South Korean sailors died.

North Korea is examining what harsher measures it would take against Gomes under a wartime law, and would be compelled to consider applying the law if the U.S. persists in its "hostile approach," the official Korean Central News Agency reported Thursday.

"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," KCNA said.

The U.S. and North Korea do not have diplomatic relations. The 1950-53 Korean War ended in a cease-fire, not a peace treaty.

American journalists Laura Ling and Euna Lee were held for five months before North Korea released them last August, and activist Robert Park was expelled about 40 days after illegally crossing into North Korea last Christmas.