Filed at 9:04 p.m. ET

SEOUL, Aug 4 (Reuters) - Former U.S. President Bill Clinton is on his way to North Korea to try to negotiate the release of two American journalists convicted by the communist state of "grave crimes", South Korea's Yonhap news agency said on Tuesday.

Clinton had already left for the North but had not yet arrived in Pyongyang, Yonhap said in a report from Washington quoting a source familiar with the issue.

"As soon as he arrives, he will be entering negotiations with the North for the release of the female journalists," the source was quoted as saying.

South Korean Foreign Ministry officials declined to comment, saying any announcement would come from Washington or Pyongyang.

The journalists were sentenced last month to 12 years' hard labour by the North after they were arrested at the border with China in March, accused of illegal entry and being "bent on slander".

The journalists, Euna Lee and Laura Ling, of U.S. media outlet Current TV co-founded by former U.S. Vice President Al Gore, were arrested when working on a story near the border between North Korea and China.

Ling has told her sister by telephone that she and Lee broke North Korea's law. U.S. Secretary of State Hillary Clinton, the former first lady, has urged Pyongyang to grant them amnesty.

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