North Korea's relationship with South Korea dipped further when a Seoul court jailed two North Koreans for plotting to assassinate a high-ranking North Korean defector.

South Korean prosecutors wanted 15-year sentences, but the court jailed the two men, identified only as Kim and Tong, both age 36, for 10 years each.

Kim and Tong admitted during their trial that they pretended to be defectors and had a secret mission to kill Hwang Jang-jop, a fellow North Korean and a senior member of North Korea's ruling Workers' Party.

Hwang defected to South Korea in 1997 and has been living in an undisclosed location under heavy police protection because of his ardent criticism of North Korea's leadership.

A lesser sentence was handed down because the accused had no choice to do other than what they were told, a judge said. The two men also cooperated fully with the investigation.

"The defendants admit all the charges, and testimony from other defectors and former agents verified their statements," said a senior judge. "Had the defendants succeeded in taking root here, they would have posed a very serious threat to Hwang's life."

North Korea repeatedly denied the existence of the plot to kill Hwang.

The denial comes as North Korea, still technically at war with the south, continues to deny any involvement in the sinking in March of the South Korean navy's 1,200-ton patrol ship Cheonan in which 46 sailors lost their lives.

The blast was strong enough to break the ship in half. It had to be salvaged from the disputed shallow waters off the west coast near the 1953 demarcation line created by a cease fire between the two warring Koreas.

An international investigating team said it had found strong evidence that the Cheonan was sunk by a torpedo of North Korean manufacture. The investigating committee believed the torpedo was fired by a small to mid-size submarine.

But North Korea's National Defense Commission immediately issued statements denying it had anything to do with the attack on the Cheonan, part of the South Korean "puppet navy."

U.N. Secretary-General Ban Ki-moon said the report was "deeply troubling" and the U.S. government described the sinking as an "act of aggression" that threatens peace in the region.

South Korean President Lee Myung-bak pledged at the time to take "stern action" and said he would be taking the incident to the U.N. Security Council, of which China is a permanent member along with the United States.

A recent joint statement in Canada by leaders of the G-8 countries, the world's eight richest nations, criticized North Korea's attack.

The G-8 members are the United States, Britain, Canada, France, Germany, Italy, Russia and Japan. Four of them -- the United States, Britain, France and Russia -- are among the five permanent members of the Security Council.

But North Korea, in a letter to the president of the United Nations Security Council this week, called for a new, joint North Korean and South Korean investigation into the sinking.

Seoul and the United States were quick to reject the North Korean proposal.

The Cheonan incident is not likely buried yet. A team of Russian experts visited South Korea last month to review the results and evidence considered by the previous international investigators.

The Russian team's report, expected soon, could work in favor of the North Koreans if it successfully casts doubt on the original investigating team's findings.

China, North Korean's staunchest ally and the fifth permanent member of the U.N. Security Council, is said to be losing patience with North Korea concerning its protestations over its alleged involvement while not backing them up with any hard evidence to the contrary.