A South Korean military court on Tuesday handed down a death sentence to a young conscript who killed five colleagues in a shooting spree at a guard post near the border with North Korea last year.

The court martial in the eastern city of Wonju condemned the 22-year-old sergeant, surnamed Lim, to death for committing what it called "premeditated and cruel" killings, Yonhap news agency said.

North Korea's leader Kim Jong-Un looks set to take an initial, tentative step onto the global stage with a visit in May to Russia -- his first trip abroad since coming to power three years ago.

Kim may not have the physique or bearing of a shy debutante, but Moscow will be his diplomatic "coming out" and minutely scrutinised, especially with other world leaders, including China's Xi Jinping, expected to be there.

Recent satellite images suggest North Korea may be about to restart the nuclear reactor seen as its main source of weapons-grade plutonium, a U.S. think-tank said Thursday.

When fully operational, the five-megawatt reactor at the Yongbyon nuclear complex is capable of producing around six kilos (13 pounds) of plutonium a year -- enough for one nuclear bomb, experts say.

The Kremlin on Wednesday confirmed that North Korean leader Kim Jong-Un will attend World War II ceremonies in Moscow in May, in what would be his first foreign trip since taking power in 2011.

"The participation of the North Korean leader has been confirmed, we are preparing for his arrival," President Vladimir Putin's spokesman Dmitry Peskov told the Interfax news agency. 

South Korea said Tuesday it had blocked a North Korean bid for the right to detain South Korean businessmen working in the Kaesong joint industrial zone in the event of a dispute.

The Kaesong complex, which lies about 10 kilometres (six miles) inside North Korea, hosts some 100 Seoul-owned factories where 53,000 North Korean workers produce goods from clothes to watches. 

North Korea Sunday described U.S. President Barack Obama as a "loser" over his criticism on its regime, accusing him of being obsessed with hostility towards Pyongyang.

The comment from the North's foreign ministry came after Obama spoke of the eventual collapse of the regime ruling what he called "the most isolated, the most sanctioned, the most cut-off nation on Earth". 

North Korea has told the United Nations that its call to refer Pyongyang to the International Criminal Court is invalid after a prominent defector retracted part of his testimony to a U.N. rights inquiry.

North Korea made the argument in a letter dated Wednesday from Ambassador Ja Song Nam to the U.N. General Assembly and Secretary-General Ban Ki-moon.

China would welcome a visit by Kim Jong-Un to Moscow, a government spokeswoman said Thursday, amid speculation that the North Korean leader could make his international debut in the Russian capital later this year.

May 9 marks the 70th anniversary of the Soviet Union's victory over Nazi Germany in World War II, and around 20 foreign leaders, including Chinese President Xi Jinping, are expected to attend a commemoration event in Moscow.

North Korea threatened Wednesday to reconsider proposals for dialogue with South Korea, condemning this week's propaganda balloon launch by U.S. and South Korean activists as an "intolerable" provocation.

The threat came two days after North Korean defectors led by Park Sang-Hak teamed up with activists from the U.S.-based Human Rights Foundation to launch some 100,000 anti-Pyongyang leaflets over the border by balloon.

North Korea argued Wednesday that the admission of inaccuracies in the memoir of a high-profile gulag survivor rendered any existing or future U.N. resolution on Pyongyang's human rights record "invalid".

Defector Shin Dong-Hyuk acknowledged this week that some elements of his story as told in the best-selling book "Escape from Camp 14" were inaccurate, although he stressed that the crucial details of his suffering and torture still stood.