South Korean activists on Monday launched tens of thousands of anti-Pyongyang leaflets into North Korea for the second time in three days, amid heightened military tensions on the divided peninsula.

The conservative activists, including many North Korean defectors, used gas-filled balloons to float 100,000 leaflets criticizing North Korean leader Kim Jong-Un over the heavily militarized border.

A North Korean submarine is missing, reports said Saturday, as the reclusive state issued a fresh threat of retaliation against U.S. and South Korean forces involved in joint military drills.

The unknown class of vessel had been reportedly operating off the North Korean coast earlier in the week when it disappeared.

The U.N. commissioner who investigated human rights in North Korea on Friday recommended establishing a panel of experts to study how crimes against humanity in the reclusive state can be punished.

"What do you do if we bring home powerful evidence of crimes against humanity -- and a veto" from one of the five permanent members of the United Nations Security Council "prevents you taking it further? How can you deal with that problem?" said Michael Kirby at a conference in London on human rights in North Korea.

The Russian foreign ministry on Monday condemned as "totally unacceptable" North Korea's threats to launch preemptive nuclear strikes against South Korea and the United States, and appealed to all sides to show restraint.

North Korea's threat of "indiscriminate" strikes came just hours before South Korea and the U.S. began annual, large-scale military drills, which usually provoke a hardline response from Pyongyang.

The U.N. Security Council unanimously adopted on Wednesday the toughest sanctions ever imposed on North Korea in response to its fourth nuclear test and rocket launch.

The 15-member council passed a resolution drafted by the United States and backed by China, Pyongyang's sole ally, that was seen as a major shift in the approach to North Korea's provocative actions.

South Korea's parliament on Wednesday passed long-delayed legislation addressing the human rights situation in North Korea -- a move likely to anger Pyongyang with tensions already running high on the divided Korean peninsula.

The bill, first proposed in 2005 but stalled for more than a decade by rival party bickering, has modest aims, including funding for civil activist groups and the creation of an official archive to track and detail rights abuses in the North.

North Korea on Saturday boasted of a newly developed anti-tank weapon that its leader said was so powerful it could turn the most heavily armored enemy tanks into "boiled pumpkin".

Pyongyang's state media said leader Kim Jong-Un had watched tests of the portable, laser-guided rocket and declared it had the "longest firing range in the world", and was "as accurate as a sniper's rifle".

North Korea on Tuesday lashed out at an upcoming joint U.S.-South Korean military exercise, warning it would attack the South and the U.S. mainland in case of any armed provocation.

The South and its close U.S. ally will next month hold their largest-ever annual exercise in response to the North's recent nuclear test and long-range rocket launch, Seoul's defense ministry has announced.

North Korean state media on Sunday confirmed the country has a new military chief following earlier reports in Seoul that the former holder of the post had been executed.

Ri Myong-Su, former People's Security Minister, was referred to as "chief of the Korean People's Army General Staff" when the Korean Central News Agency (KCNA) reported on an army exercise guided by leader Kim Jong-Un.

North Korea is committed to striking the United States with a nuclear-armed missile, but it can't do so without outside help, due to shortfalls in its own technology, the Pentagon said Friday.

The report to Congress was written prior to a fourth nuclear test conducted by Pyongyang last month and the launch of a satellite-bearing rocket earlier this month.