SEOUL - Two US women journalists will go on trial in North Korea's highest court Thursday on charges that could send them to a labour camp, amid growing international tensions sparked by Pyongyang's nuclear test.

The pair were detained by North Korean border guards on March 17 along the narrow Tumen River which marks the border with China, while researching a story about refugees fleeing the hardline communist state.

Pyongyang has said they will face trial for "hostile acts" and illegally entering the country, with the hearing to be held "on the basis of the confirmed crimes committed by them."

South Korean analysts say "hostile acts" are punishable by a minimum five years' detention and hard labour.

The families of Euna Lee and Laura Ling Monday broke their long silence to appeal for clemency and to urge the two governments not to link the case to the nuclear standoff.

The reporters, who work for California-based Current TV co-founded by former vice president Al Gore, were allowed to phone their families in the US a week ago.

"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.

"Now is the time to try and urge both governments to communicate," Lisa told NBC's "The Today Show".

Both detainees are married and Lee has a four-year-old daughter. Lisa Ling said Laura had a recurring ulcer but was being allowed to take medication.

Sweden's ambassador to Pyongyang Mats Foyer, who represents US interests in the absence of diplomatic relations, met the pair Monday for the third time since they were detained.

Laura Ling, in a letter to her sister released on the social networking website Facebook, said she had "cried so much" during her imprisonment but was gradually feeling better.

"I try very hard to think about positive things, but sometimes it is hard to," she wrote in the letter. The family said the letter, delivered via the Swedish ambassador, was dated May 15.

Ling said some days she was allowed to go outside for fresh air.

"I also sit and meditate. I breathe deeply and think about positive things that have happened in the day," she wrote.

International rights and media freedom groups as well as Western politicians have urged Pyongyang to release the reporters.

US Secretary of State Hillary Clinton has said the charges they face are "baseless."

Analysts have said Pyongyang may be using the pair as a bargaining chip to open direct talks with Washington, possibly following the lead set by Tehran.

US-born reporter Roxana Saberi walked free from an Iranian jail last month after a court reduced her eight-year prison term for spying to a two-year suspended sentence.

But relations between Washington and Pyongyang have worsened in recent weeks, with the North firing 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 on May 25 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.

Supporters of the two reporters plan rallies in Washington and other US cities late Wednesday to coincide with the trial.

A support group said the rallies would make the point that if the journalists entered North Korea, it would have been by accident and they were ready to apologise. -AFP