North Korea tested a nuclear weapon on 25 May, which is claimed to be as powerful as the Hiroshima atomic bomb.
The test comes less than two months after the North enraged the US and its allies by test firing a long-range ballistic missile.
Russian defence experts estimated the explosion's yield at between 10 and 20 kilotons, as powerful as the US bombs against Hiroshima and Nagasaki.
The force of the blast made the ground tremble in the Chinese border city of Yanji, 130 miles away.
South Korean officials detected a tremor similar to that caused by an underground nuclear explosion.
The country also test fired three short-range missiles from a base on the east coast immediately after the nuclear test, the country's Yonhap news agency reported.
The nuke test received strong condemnation from the UN and countries like the US, Russia, China, South Korea and Japan.
US President Barack Obama said that North Korea's behaviour would increase tensions and undermine stability in north-east Asia.
"It will not find international acceptance unless it abandons its pursuit of weapons of mass destruction and their means of delivery," said Obama.
North Korea conducted its first nuclear test in 2006 that measured 1 kiloton.
The country had warned of a second nuclear test after the UN condemned its test launch of a ballistic missile on 5 April and agreed to tighten sanctions put in place in 2006.