Image caption Chang Song-thaek, left, is believed to have mentored Kim Jong-un

North Korean leader Kim Jong-un has spoken of the "elimination of factionalist filth" in his first public reference to the execution of his once-powerful uncle last month.

In a new year message broadcast on state TV, he said "resolute action" had strengthened the country's unity.

North Korea announced on 13 December it had executed Chang Song-thaek after convicting him of "acts of treachery".

The move raised concerns of instability in the secretive nuclear-armed country.

North Korea's new year messages are closely scrutinised by South Korea and other regional powers for clues to Pyongyang's policy goals.

Kim Jong-un said action to "eliminate factionalist filth" within the ruling Workers' Party had bolstered the country's unity "by 100 times".

'Warmongers'

"Our party's timely, accurate decision to purge the anti-party, anti-revolutionary elements helped greatly cement solidarity within our party," he said.

Image caption Chang Song-thaek was given a military trial and then executed immediately

Mr Kim also accused South Korea and US of being "warmongers", working "frantically" to bring nuclear weapons to the peninsula.

An accidental conflict, he said, could trigger "an enormous nuclear catastrophe".

Analysis of Kim's speech Kim Jong-un's address was aired four times, at regular intervals. It was 26 minutes long, but Mr Kim is only seen in vision for approximately three-and-a-half minutes. The speech is punctuated by regular and uniform applause - during which the facade of the Korean Workers' Party Central Committee Headquarters building is shown. The audience is never seen. Although no other part of the room or hall where the address is being made is seen, sound quality is clear with a distinct echo. An analysis of the audio by BBC Monitoring suggests that the same clip of applause is used several times throughout the broadcast. It is impossible to ascertain whether an audience was present or not, and an official photo published by the official KCNA news agency is closely cropped, showing Mr Kim standing at the dais. BBC Monitoring

He called for the strengthening of North Korea's defence capabilities, adding that the country's dignity and the people's happiness depend on "the gun barrel".

Mr Chang was married to the sister of late leader Kim Jong-il, and is believed to have mentored Kim Jong-un when he succeeded his father in 2011.

Although seen as the second-most important figure in the country, he was dramatically removed from a special party session by armed guards and stripped of all his titles.

The state news agency KCNA later said he had admitted at a military trial to trying to overthrow the state, and had been executed immediately.

Analysts said one theory for Mr Chang's downfall was that he was too keen an admirer of China's economic reform.

The 1950-53 Korean War ended with an armistice, not a peace treaty, leaving the peninsula technically in a state of war.

About 28,500 US troops are deployed in South Korea to help deter North Korean aggression.