These are euphoric but anxious days for South Korea, as the heady impeachment of a deeply unpopular president leaves the country without a recognized leader at a time of military tensions with nuclear-armed North Korea.

The United Nations Security Council on Wednesday unanimously imposed its toughest ever sanctions on North Korea, placing a cap on its key coal exports after the state's defiant nuclear tests.

The new sanctions resolution, which was spearheaded by the United States and came after three months of tough negotiations with fellow veto-wielding council member China, passed by a 15-0 vote.

Convincing North Korea to abandon nuclear weapons is a "lost cause," America's top intelligence official said Tuesday, causing concern in the State Department and ally South Korea over an issue of long-standing U.S. policy.

The United States has always maintained it cannot accept North Korea as a nuclear state and, under President Barack Obama, has made any talks with the North conditional on Pyongyang first making some tangible commitment towards denuclearisation.

The U.N. Security Council on Monday denounced North Korea's latest test of a powerful missile that one leading US expert warned could be put into operational service as early as next year.

North Korea test-fired the medium-range Musudan -- capable of hitting U.S. bases as far away as Guam -- on Saturday.

A North Korean soldier walked across one of world's most heavily fortified borders on Thursday and defected to the South, military authorities said.

Flooding following heavy rain has killed 60 people and left over 44,000 homeless in North Korea, the United Nations said Tuesday, after the country reported that a northeastern river suffered its worst-ever flood.

South Korea and the United States kicked off large-scale military exercises on Monday, triggering condemnation and threats of a pre-emptive nuclear strike from North Korea.

North Korea has accused Washington of planning a pre-emptive nuclear strike, after the U.S. announced it would deploy its B-1 bomber in the Pacific for the first time in a decade.

The strategic aircraft were to be deployed on Saturday on the U.S. island of Guam, the U.S. military said last month, describing the operation as a routine rotation with the B-52 bomber.

The United States warned North Korea Tuesday that it faces "real consequences" over its internationally condemned nuclear and missile tests, which have inflamed tensions in East Asia.

North Korea said Wednesday its latest ballistic missile tests trialled detonation devices for possible nuclear strikes on U.S. targets in South Korea and were personally monitored by supreme leader Kim Jong-Un.