The two were sentenced on Monday following a trial behind closed doors that began last week, the North's official Korean Central news agency (KCNA) said.

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"The trial confirmed the grave crime they committed against the Korean nation and their illegal border crossing as they had already been indicted and sentenced each of them to 12 years of reform through labour," KCNA said, giving no other details.

The two journalists who were working for California-based Current TV, an online service set up by Al Gore, the former US vice-president, cannot appeal because they were tried in the country's highest court, where decisions are final.

North Korea has said the two reporters had crossed illegally to its side of the border with China, although other reports have suggested the two were on the Chinese side when they were arrested.

Previous reports from North Korea have said Ling and Lee were accused of "hostile acts" but also did not give details.

Tensions with US

Their arrest and trial come against a background of heightened tensions on the Korean peninsula, following the North's recent nuclear and missile tests.

"The trial confirmed the grave crime they committed against the Korean nation ... they had already been indicted and sentenced each of them to 12 years of reform through labour"

North Korea's KCNA news agency

Analysts say the sentences appeared unusually harsh and seemed to back up views that the journalists could be used as a bargaining chip by the North in its standoff with the US.

News of the verdict came a day after Hillary Clinton, the US secretary of state, said Washington was looking into putting North Korea back on a list of state sponsors of terrorism, a designation that could subject the impoverished state to more financial sanctions.

The US removed North Korea from the blacklist in October in a bid to revive faltering six-party nuclear disarmament talks, prompting the North to take some measures to disable its nuclear facilities.

But Pyongyang has since reversed those steps and said it had restarted its nuclear complex, including reprocessing nuclear fuel to obtain weapons-grade plutonium.

Clinton appealed on Sunday for the two women's release, saying their case was a humanitarian issue and separate from the nuclear issue.

Pyongyang has also vowed to retaliate with "extreme" measures if the United Nations punished it for conducting its nuclear test last month.

The security council has been deliberating its response and may issue a new resolution as early as this week, although there appear to be division among members over how tough sanctions should be.

"Our response would be to consider sanctions against us as a declaration of war and answer it with extreme hardline measures," the North's official Rodong Sinmun newspaper said in a commentary on Monday.