SEOUL – North Korea said on Tuesday that two U.S. journalists it sentenced to 12 years of hard labor were trying to slander the state, giving its first detailed account of the case that worsened ties with the United States.

One of North Korea's top courts, in a trial closed to outside observers, last week convicted and sentenced Laura Ling and Euna Lee, both in their 30s and working for U.S. media outlet Current TV, saying they illegally entered the country to commit grave crime.

At the trial the accused admitted that what they did were criminal acts committed, prompted by the political motive to isolate and stifle the socialist system of the DPRK (North Korea) by faking up moving images aimed at falsifying its human rights performance and hurling slanders and calumnies at it, the official KCNA news agency said.

We are following with a high degree of vigilance the attitude of the U.S. which spawned the criminal act against the DPRK, KCNA said.

The U.S. government has demanded their release with Secretary of State Hillary Clinton calling the charges baseless.

Analysts say the North is using the journalists as bargaining chips with long time adversary the United States, which for years has tried to end Pyongyang's pursuit of atomic weapons.

South Korean President Lee Myung-bak will work to form a united policy on North Korea when he meets President Barack Obama on Tuesday in Washington.

Human rights groups denounced the trial and said the North's judicial system is designed to protect the unquestioned rule of leader Kim Jong-il and not the rule of law.

KCNA said the two will not be able to appeal against their sentence.

KCNA said the two crossed the Tumen River at dawn on March 17 into North Korea with two men to make a documentary slandering the North. It named U.S.-based members of the Current TV staff it said were a part of a conspiracy.

Details of their arrest, including the actual spot where they were taken into custody, are still sketchy. They were arrested near the border with China.

The United States and other governments have accused North Korea of massive human rights abuses, saying it maintains a network of political prisons to stamp out dissent and holds public executions to intimidate the masses.

(Editing by David Fox)