South Korea on Friday dismissed North Korean leader Kim Jong-Un's conciliatory words in his New Year message as an empty gesture, urging Pyongyang to scrap its nuclear programmes to show it is committed to better relations.

In his speech on Wednesday, Kim hailed the execution last month of his once-powerful uncle Jang Song-Thaek and accused the United States and South Korea of manoeuvring for a nuclear war.

But he also called for a "favourable climate" to improve relations with the South, saying it was time for the two Koreas to stop doing "anything detrimental to national unity and reconciliation".

In its first official response, the South Korean government said it was sceptical about the intentions of Kim, who has ruled the nuclear-armed North since the death of his father, Kim Jong-Il, in December 2011.

"Peace and reconciliation cannot be achieved merely by words", Seoul said in a statement.

"In order to improve ties between the South and the North, North Korea must show sincerity in building trust and above all, it must make genuine efforts for denuclearisation".

It said Kim made similar comments in last year's New Year speech before a series of provocative actions from the North including a third nuclear test, threats of military attacks and the unilateral closure of an inter-Korean industrial zone.

Pyongyang shut down the complex at Kaesong in April during a spike in military tensions that followed the nuclear test but the two Koreas agreed in September to resume operations.

South Korean Defence Minister Kim Kwan-Jin on Thursday cautioned that the apparent peace overtures from the North could be a "smoke screen" aimed at hiding a provocative act, urging the military to remain alert.