|Kim Jong-un, right, has been appointed supreme commander of the military and ruling party leader [Reuters]
North Korean troops have shot dead three compatriots who were trying to cross the border into China, according to a South Korean activist.
The report comes as North Korea moves to tighten border controls after the death of the leader, Kim Jong-il.
Border guards killed the men in their 40s who were crossing the Yalu river from the northern border city of Hyesan on Saturday, Do Hee-youn, who helps refugees from the North, said.
"People waiting at the Chinese side across the river to help the three defect saw the scene. The guards took with them the bodies which were lying on the ice," Do told AFP news agency on Tuesday, citing sources in China's border county of Changbai.
South Korea's intelligence service said it could not immediately confirm the reported shootings.
Activists say the North has intensified patrols along its border to try to prevent defections in the wake of Kim's death on December 17.
They fear a tougher crackdown during the politically sensitive transition which sees Kim Jong-un, the son of the late leader, take power as supreme commander of the military and the ruling party leader.
"They are trying to let people know that those trying to flee will be shot dead right away," Do said, citing sources in the North who communicate via mobile phones smuggled in from China.
About 23,000 North Koreans have fled repression or hunger in their homeland for South Korea since the 1950-1953 war, the vast majority in recent years.
Image of strength
Al Jazeera's Harry Fawcett, reporting from Seoul on Monday, said: "Everything the North Korea is doing at the moment is designed to project stability and continued power and strength.
"Any movement toward anything which might be interpreted as a weakness will be avoided."
He was referring to a New Year's statement by the North on Sunday in which it said it would bolster its military and defend Kim Jong-un "unto death".
North Korea said on Friday that there would be no softening of its position towards South Korea's government after Kim Jong-il's death.
Cross-border ties have been frosty since the South accused the North of torpedoing one of its warships in March 2010 with the loss of at least 46 lives.
The North denied involvement but killed four South Koreans in an artillery attack on a border island in November 2010.
Disarmament for aid
North Korea has regularly criticised South Korea's President Lee Myung-bak since he took office in 2008 and ended a no-strings-attached aid policy towards the North. Lee has sought to link aid to progress in North Korea's nuclear disarmament.
North Korea's powerful National Defence Commission said the country would never deal with Lee.
However, Lee pushed on Monday for a speedy resumption of nuclear talks, which were under way and said to have been making advances at the time of Kim Jong-il's death.
"The situation on the Korean peninsula is now entering a new turning point," Lee said in a nationally televised speech. "But there should be a new opportunity amid changes and uncertainty."
He said that if North Korea demonstrated sincerity, a new era on the Korean peninsula could be opened, but said that South Korea would sternly respond to any attack from the North.
Lee reasserted that if North Korea halted its continuing nuclear activities, there could be a resumption of international disarmament-for-aid negotiations meant to rid the North of its nuclear programmes.