U.N. investigators probing human rights abuses in North Korea heard harrowing evidence in London on Wednesday from people who have managed to flee the secretive Stalinist regime.

The landmark U.N. rights commission heard from a handful of defectors who have reached Europe after similar hearings were held in Seoul and Tokyo.

North Korea on Monday warned of "merciless firing" against the South if it goes ahead with a reported plan to develop shells to carry anti-Pyongyang leaflets across the border.

The South's Joong-Ang Ilbo newspaper reported last week that South Korean troops were developing non-explosive hollow shells capable of carrying such leaflets deep into North Korean territory. There has been no official confirmation.

North Korea on Saturday issued a fresh warning of an "all-out war", urging the United States to stop military drills and what it described as "nuclear blackmail.”

In a thinly veiled threat to strike the United States, the North's National Defense Commission (NDC), chaired by leader Kim Jong-Un, said the U.S. government must withdraw its policy of hostility against the North if it wants peace on both the Korean peninsula and the "U.S. mainland.”

North Korea's leader Kim Jong-Un inspected a set of newly built warships and issued orders to bolster the navy, state media said Saturday.

The visit followed a two-day joint naval drill near the Korean peninsula involving a U.S. nuclear aircraft carrier which sparked a series of angry responses and threats from Pyongyang.

North Korea on Friday threatened to "bury in the sea" a U.S. aircraft carrier, as it slammed a three-nation naval drill involving U.S., South Korean and Japanese warships.

The latest warning from the isolated regime came a day after the United States launched a two-day joint military drill with South Korea and Japan off the southern coast of the Korean peninsula.

North Korean state media confirmed Thursday that the nation's hawkish, aging army chief has been replaced in a reshuffle that observers say sought to tighten leader Kim Jong-Un's grip over the military.

Rumors had circulated in August that Kim Kyok-Sik, 75, had been replaced as chief of the army's general staff by Ri Yong-Gil.

The United States launched a joint naval drill Thursday with South Korea and Japan despite dire warnings of a "horrible disaster" from North Korea, officials said.

The two-day drill involving a U.S. nuclear-powered aircraft carrier alongside South Korean and Japanese vessels began off the southern coast of the Korean peninsula, the South's defense ministry said.

South Korea's spy chief has said he is "aware" of the alleged execution of members of the North's national orchestra, said to include a singer rumored to be leader Kim Jong-Un's former girlfriend.

Several members of the orchestra and other state music troupes had been executed by firing squad for taping themselves having sex, Japan's Asahi Shimbun said last month in a report that was picked up by South Korean broadcasters and websites.

South Korea's spy agency confirmed Tuesday that the North has restarted an aging plutonium reactor that could help boost its nuclear weapons program.

The National Intelligence Service (NIS) said in a report to parliament that the five megawatt reactor at the North's Yongbyon nuclear complex had resumed operations, according to a joint briefing by ruling and opposition party lawmakers.

North Korea on Tuesday warned the United States of a "horrible disaster" and put its troops on alert over a massive joint naval drill involving a nuclear-powered U.S. aircraft carrier alongside South Korean and Japanese vessels.

The warning came after Seoul and Washington last week signed a new joint strategy to counter the growing threat of a North Korean nuclear attack after the communist country restarted an ageing plutonium reactor.