SEOUL - A North Korean court Monday sentenced two female US journalists to 12 years in a labour camp for an  illegal border crossing and an unspecified "grave crime," state media reported.

The "trial confirmed the grave crime they committed against the Korean nation and their illegal border crossing," the Korean Central News Agency said.

It said the court, after a five-day trial, "sentenced each of them to 12 years of reform through labour."

The sentences were certain to fuel tensions with Washington after the North's May 25 nuclear test and its reported plans for another long-range rocket launch.

US Secretary of State Hillary Clinton said Sunday the charges against the pair were baseless and they should be allowed to return home.

Clinton also said the United States is considering putting North Korea back on its terrorism blacklist following its nuclear test.

Laura Ling and Euna Lee were detained by North Korean border guards on March 17 along the frozen Tumen River, which marks the border with China, while researching a story about refugees fleeing the hardline communist state.

Pyongyang had previously said they would face trial for "hostile acts" and illegal entry but never gave details of such acts.

Analysts have said the women may become pawns in efforts to open direct negotiations with the United States.

The North has long sought direct talks in preference to the stalled six-party negotiations on nuclear disarmament of which Washington is a part.

The sentences "are tougher than expected," said Yoo Ho-Yeol, a professor and North Korea expert at Korea University.

"They aim to send a strong message to the United States that the North is taking the case very seriously and gravely," he told AFP.

"It also shows the North's strategy to bring the United States to the bilateral negotiating table."

Pyongyang has in the past freed captured Americans but only after personal interventions. The US State Department last week did not rule out the possibility that former vice president Al Gore might undertake such a mission.

Gore is chairman of the California station Current TV, which employs the two journalists, both aged in their thirties.

Both detainees are married and Lee has a four-year-old daughter.

Friends, family and colleagues held candlelight vigils in Washington and seven other US cities last week. Their families have appealed for clemency and urged the two governments not to link the case to the nuclear standoff.

Sweden's envoy in Pyongyang, who represents US interests in the absence of diplomatic ties, has been allowed three visits to the women. The North on May 26 allowed them to phone their families in the United States.

"We had not heard their voices in over two and a half months," said Ling's sister Lisa. "They are very scared - they're very, very scared."

The North has been showing an increasingly defiant face to the world since it fired a long-range rocket on April 5 despite international appeals to refrain.

After the United Nations Security Council punished the launch by tightening sanctions, the North responded with its second nuclear test.

It has also renounced the armistice on the Korean peninsula and is said to be preparing to test medium-range missiles and a long-range Taepodong-2.

The North is also holding a South Korean employee of the Kaesong joint industrial estate just north of the border.

He has been detained since March 30 for allegedly criticising Pyongyang's political system and encouraging a woman worker to defect.

The North has accepted the South's proposal for working-level talks next week to settle disagreements over Kaesong's operations. But analysts said this did not indicate any desire to improve ties with Seoul.