North Korea has threatened a "merciless" strike against the South after activists burned effigies of the ruling Kim dynasty on the second anniversary of the death of former leader Kim Jong-Il, officials said Friday.

The warning was contained in a message sent Thursday by the secretariat of the National Defence Commission, the North's highest military body, through a military hotline, the South's defence ministry said.

In rallies on Tuesday to mark the death anniversary of Kim Jong-Il, South Korean conservative groups burned effigies of young North Korean leader Kim Jong-Un, his late father and grandfather.

The North said the rallies had insulted the "highest dignity" of its leadership, and threatened to take "merciless" retaliatory acts without prior warning, the defence ministry said.

"We've sent a reply vowing to react sternly to any provocations by North Korea," ministry spokesman Kim Min-Seok told reporters.

The North's threat comes at a time of growing concern over the regime's stability after last week's execution of Jang Song-Thaek, a high-level official who was the uncle and former political mentor of Kim Jong-Un.

Seoul and Washington have warned of possible provocative acts by the nuclear-armed North.

North Korea has a long history of bellicose rhetoric, regularly threatening strikes against South Korea.

Tensions between the two Koreas had appeared to cool down after soaring in February, when the North carried out its third underground nuclear test in defiance of UN Security Council resolutions.

But relations have soured again in recent weeks.

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  • North Koreans dance and flip colored cards to form a giant image in the seats of a stadium during an Arirang mass games performance in Pyongyang, North Korea, on Sunday, Sept. 9, 2012. (AP Photo/David Guttenfelder)

  • Thousands of North Koreans participate in the "Arirang" mass games in Pyongyang, Monday, July 22, 2013. North Korea has revealed a new rendition of the "Arirang" mass games, the song-and-dance ensemble. (AP Photo/Wong Maye-E)

  • The May Day stadium is enveloped in a layer of mist after torrential rain in Pyongyang, Monday, July 22, 2013. (AP Photo/Wong Maye-E)

  • North Korean children perform during the "Arirang" mass games on Monday, July 22, 2013 in Pyongyang, North Korea. (AP Photo/Wong Maye-E)

  • North Korean performers gather for the finale of their "Arirang" mass games on Monday, July 22, 2013 in Pyongyang, North Korea. (AP Photo/Wong Maye-E)

  • North Korean dancers perform during the "Arirang" mass games, in Pyongyang, Monday, July 22, 2013. (AP Photo/Wong Maye-E)

  • North Korean women jump through hula hoops during the opening night of the "Arirang" mass games performance at Pyongyang's May Day Stadium on Monday, July 22, 2013. (AP Photo/David Guttenfelder)

  • North Korean girls perform with hoops on Monday, July 22, 2013 in Pyongyang, North Korea. (AP Photo/Wong Maye-E)

  • Performers participate in the Arirang mass games in Pyongyang, Monday, July 22, 2013. (AP Photo/Wong Maye-E)

  • North Korean women dance in front of a display of their country's national flag on Monday, July 22, 2013 in Pyongyang, North Korea. (AP Photo/Wong Maye-E)

  • Performers dance in front of portraits of late North Korean leaders Kim Il Sung, left, and Kim Jong Il, right, in Pyongyang, Monday, July 22, 2013. (AP Photo/Wong Maye-E)

  • North Koreans stand to applaud during the opening night of the "Arirang" mass games performance at Pyongyang's May Day Stadium on Monday, July 22, 2013. (AP Photo/David Guttenfelder)

  • North Koreans stand to applaud during the opening night of the "Arirang" mass games performance at Pyongyang's May Day Stadium, Monday, July 22, 2013. (AP Photo/David Guttenfelder)

  • Thousands of North Koreans turn colored cards to create an image of the Unha-3 rocket that North Korea successfully launched in December of 2012 during the opening night of the Arirang mass games at Pyongyang's May Day Stadium, Monday, July 22, 2013. (AP Photo/David Guttenfelder)

  • North Koreans children dance during an Arirang mass games performance in Pyongyang, North Korea, on Sunday, Sept. 9, 2012. (AP Photo/David Guttenfelder)

  • Performers march during the "Arirang" mass games in Pyongyang, Monday, July 22, 2013. North Korea has revealed a new rendition of the "Arirang" mass games, the song-and-dance ensemble. (AP Photo/Wong Maye-E)