12 January 2016, 07:29 GMT
MOSCOW (Sputnik) – On January 6, Pyongyang claimed it had carried out a hydrogen bomb test. Seoul started anti-North propaganda broadcasting through loudspeakers over the border in response.
"We are closely monitoring the situation," the source told RIA Novosti.
North Korea declared itself a nuclear power in 2005 and conducted nuclear tests in 2006, 2009 and 2013, having earlier withdrawn from the Nuclear Non-Proliferation Treaty that it ratified in 1985.
South and North Korea are legally at war, as no peace treaty was signed after the Korean War of 1950-1953.
