South Korean protesters shout slogans during a rally in Seoul on June 9, demanding release of the American journalists detained in North Korea. (Ahn Young-joon/Associated Press)

Two American journalists sentenced by North Korea last week to 12 years of hard labour for politically motivated acts were caught filming their illegal crossing into the country, state-run media said Tuesday, providing the first details on the arrests.

The reporting team from Current TV crossed the frozen Tumen River dividing North Korea and China on March 17 and walked up the river bank — all the while recording their transgression, the official Korean Central News Agency said.

"We've just entered a North Korean courtyard without permission," the Korean translation of their narration on the videotape said, according to the agency. One of them picked up and pocketed a stone as a memento of the illegal move, the report said.

Two women — reporter Laura Ling, 36, and editor Euna Lee, 32 — were arrested in Kangan-ri in North Hamgyong Province, the report said. A third person, Current TV executive producer Mitch Koss, and their Korean-Chinese guide managed to flee, the agency said.

On June 8, Lee and Ling were sentenced in North Korea's top court to 12 years of hard labour for what the state news agency called politically motivated crimes. They were accused of crossing into North Korea to capture video for a "smear campaign" focused on human rights, the report said.

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

The case comes at a time of rising tensions between North Korea and the United States over the Communist country's nuclear and missile programs. North Korea went ahead with a rocket launch in early April, conducted a nuclear test on May 25 and fired off a series of short-range missiles in the days before the journalists' trial.

The alleged details of the case were released by state media just hours before U.S. President Barack Obama was to sit down at the White House with South Korean President Lee Myung-bak.

The two leaders, whose countries fought together against North Korea during the 1950-53 Korean War, were expected to discuss the North and make a strong show of unity.