South Korean protesters shout slogans during a rally in Seoul on Tuesday, demanding release of American journalists detained in North Korea. (Ahn Young-joon/Associated Press)

The families of two American journalists sentenced to 12 years in a North Korean labour prison urged its hardline government to grant them clemency, amid hopes the U.S. government would send an envoy to negotiate their release.

The sentencing of Laura Ling and Euna Lee on Monday is a new challenge for U.S. President Barack Obama who last week pledged to take a "very hard look" at tougher measures against North Korea for failing to end its nuclear program and testing a second atomic device on May 25.

"We remain hopeful that the governments of the United States and North Korea can come to an agreement that will result in the release of the [women]," the families said in a joint statement released Tuesday.

"We ask the government of North Korea to show compassion and grant Laura and Euna clemency and allow them to return home to their families."

The statement expressed concern about the women's health, noting that Ling, 32, has a serious medical condition, a reference to her ulcer, and that Lee's four-year-old daughter is showing "signs of anguish over the absence of her mother."

Lee, 36, and Ling — who work for former vice president Al Gore's Current TV — were arrested March 17 near the China-North Korea border where they were reporting about the trafficking of women.

It's unclear whether they tried to sneak into the North or if aggressive border guards crossed into Chinese territory and grabbed them, as has happened before.

The North accused the reporters of unspecified "hostile acts" and illegally entering the country, but the formal charges against them were unclear. Their trial, which was closed to foreigners, began Thursday and they were sentenced Monday to 12 years of "reform through labour."

Protest in Seoul

About 200 people gathered in central Seoul on Tuesday to protest against North Korea's decision to keep the two journalists and called for their immediate release.

Analysts have said they doubt Pyongyang wants to send them to jail or to one of its gulags, where poorly fed inmates often do back-breaking work in factories, coal mines and rice paddies.

Sentencing the journalists may be a way for Pyongyang to maximize its leverage with Washington, said Roh Jeong-ho, the director of the Center for Korean Legal Studies at Columbia Law School.

"I don't think the reporters will do hard labour. It's simply not in the North Koreans' interests to make them go through that," he said in comments emailed to The Associated Press.

"Essentially, it's a whole package of brinksmanship," he said. "They want to say to the Obama administration 'take us seriously and, in turn, we'll resolve this issue for you.'"

North Korea wants to be recognized as a legitimate nuclear state but Washington has so far refused to endorse such a status for the unpredictable nation, which has a history of terrorism, ripping up agreements and sharing its nuclear know-how with nations hostile to America.

The United Nations is debating a new resolution to punish the North for its second nuclear test last month. Pyongyang followed the test with a barrage of missile launches, and is believed to be preparing another long-range missile test at a new launch pad.

'Pursuing every possible approach'

U.S. Secretary of State Hillary Rodham Clinton said Monday the Obama administration is "pursuing every possible approach that we can consider in order to persuade the North Koreans to release them and send these young women home."

She stressed that the reporters' case and Washington's efforts to punish North Korea for its recent nuclear test are "entirely separate matters."

She did not elaborate but a senior Obama administration official said New Mexico Gov. Bill Richardson and Gore had been in contact with the White House and State Department about potential next steps, including possibly sending an envoy to try to negotiate the release of the two women.