Reporters Sans Frontieres (RSF, Reporters Without Borders) also said it was "by no means clear" that the two women were on North Korean territory when they were detained by the North's border guards on March 17.

In Washington, the State Department said Tuesday it is still working with Sweden, which represents US interests in Pyongyang, in a bid to win their release.

Euna Lee, a Korean-American, and Laura Ling, a Chinese-American, were detained before dawn on March 17 along the narrow Tumen River, which marks the border with China.

The pair, who work for Current TV in California, were working on a story about refugees fleeing the hardline communist North.

The North's state media announced Tuesday it would put them on trial for "hostile acts" and illegally entering the country, further straining relations with Washington ahead of a planned rocket launch by Pyongyang.

"The illegal entry of US reporters into the DPRK (North Korea) and their suspected hostile acts have been confirmed by evidence and their statements," it said.

It added that authorities were "making a preparation for indicting them at a trial on the basis of the already confirmed suspicions."

RSF in a statement said that if convicted the pair could be sentenced to between five and 10 years of forced labour. A South Korean expert has given a similar assessment.

Some Seoul analysts say the North plans to use the pair as a bargaining chip with the United States after Pyongyang's rocket launch expected between April 4-8, and they may subsequently be freed.

Pyongyang says the rocket launch is to send up a communications satellite as part of a peaceful space programme.

The United States, Japan and South Korea say it is a pretext to test a long-range missile in violation of UN resolutions and they will refer it to the Security Council.

"Their missile launch violates UN Security Council Resolution 1718 and there will be consequences, certainly (at) the UN Security Council if they proceed with the launch," US Secretary of State Hillary Clinton said Tuesday.

RSF said it had been told by "several sources" on the Chinese side of the frontier that the North's border guards probably crossed the river while Ling and Lee were filming on the Chinese bank.

It urged a US cameraman identified as Mitch Koss, who reportedly escaped when the pair were seized and later flew home from China, to come forward and say what had happened.

"There is an urgent need for North Korea?s neighbours, especially China, to apply diplomatic pressure to obtain the release of Ling and Lee as soon as possible," RSF added.

"It would be unacceptable if North Korea used the two journalists for diplomatic blackmail at a time when it has stepped up tension in the peninsula by announcing a missile launch."

The Paris-based group said it was the first time foreign journalists have been held in the North for any length of time since the detention of Japanese reporter Takashi Sugishima from December 1999 to February 2002.

RSF also highlighted the risks North Koreans face in giving information to the media. It said Kim Sung-Chul, a member of the military, has been held since October 2006 after being identified as the person who secretly filmed video of a public execution broadcast on Japanese TV.

RSF said a North Korean TV journalist it identified as Song Keum-Chul has been detained in a camp since 1996 "for questioning the official version of certain historic events."

Copyright AFP 2008, AFP stories and photos shall not be published, broadcast, rewritten for broadcast or publication or redistributed directly or indirectly in any medium