SEOUL, South Korea - North Korea said yesterday that two American journalists it sentenced to 12 years in a labor camp admitted at their trial that they entered the country illegally to slander the North's human-rights record.

It was the first explanation from the government of Kim Jong Il of why Laura Ling and Euna Lee, detained in March while working on a story about North Koreans who flee to China, last week were given sentences that outsiders regarded as unusually harsh.

The women were arrested on the North Korean side of the Tumen River after entering the country illegally from China, the statement, released by the official Korean Central News Agency, said.

"We have just entered North Korean territory without permission," a narrator said on a video found in the reporters' possession, it said. It also said one of the reporters picked up and kept a pebble as a souvenir.

Two others in the reporting team that crossed the river, producer Mitch Koss and a Korean Chinese guide, fled, the statement said.

There have been no definitive independent accounts of where the journalists were when they were detained. Members of their families in the United States have apologized on their behalf, if the two crossed into North Korea. The U.S. government has asked for their release on humanitarian grounds.

North Korea's statement made it clear that it has found the women guilty of much more than wandering across a border.

"At the trial the accused admitted that what they did were criminal acts, prompted by a political motive to isolate and stifle the socialist system of North Korea, by faking moving images aimed at falsifying its human rights performance and hurling slanders and calumnies at it," it said.

North Korea is widely regarded as having one of the worst human-rights records in the world.

The sentence cannot be appealed, the statement said.

The incarceration of the two journalists has become part of a complex political and military crisis this year on the Korean peninsula, as North Korea has launched missiles, detonated a nuclear bomb, threatened war against its southern neighbor, and vowed never to give up nuclear weapons.

In Washington yesterday, President Obama brushed aside any suggestion that the United States would accept North Korea's desire to become a nuclear power.

"We have continually insisted that North Korea denuclearize," Obama said in the White House Rose Garden with South Korean President Lee Myung Bak at his side.

Obama said the United States, along with South Korea and regional powers such as China, Japan, and Russia, would never accept North Korea's goal of becoming a nuclear power and would break the historic cycle of paying off North Korea to stop developing nuclear weapons.

"We're going to break that pattern," the president said.

The secretive communist state, which has annoyed even its allies, China and Russia, is believed to be in the throes of a succession process, as the country's ailing leader prepares to hand power to his youngest son.

Ling, 32, and Lee, 36, work for Current TV, a cable and Web network cofounded by former Vice President Al Gore. Their five-day trial was held in Pyongyang's Central Court, the top court in North Korea. No foreign observers were allowed.

Interpreters were provided, the government statement said. It also said Ling asked for an attorney and was provided one, while Lee declined legal representation.

In the past, North Korea has eventually released Americans who have entered illegally. The government also has a history of brinkmanship, turning confrontation into negotiations that reward it with food, fuel, and other concessions.

This article includes information from McClatchy Newspapers.