North Korea has threatened to strike South Korea in response to conservatives' anti-Pyongyang rallies here on the second death anniversary of its leader Kim Jong-il earlier this week, a senior military official said Friday.

The North's powerful National Defense Commission on Thursday sent a fax to South Korea's National Security Council through the western coastal military hotline to threaten to strike the South "without notice," the official said. 

The latest threat came after several conservative groups and North Korean defectors on Tuesday held rallies in Seoul to protest against North Korea's authoritarian rule and human rights abuse, with some burning Kim Jong-un's photo.

The North Korean military condemned that the rallies insulted North Korea's "highest dignity," referring to its young leader, who had recently ordered the execution of his uncle Jang Song-thaek to consolidate his grip on power.

The South Korean government immediately replied through the military line, vowing to "sternly react" to any provocations, the official said.

North Korea has a long history of bellicose rhetoric, regularly threatening strikes against South Korea and the United States, often in response to their joint drills particularly in spring.

The North Korean military has been carrying out winter drills since early December, but it has not shown unusual movement so far, military officials said. 

The South Korean capital city of Seoul, with more than 10 million people, is within the range of North Korea's conventional artillery positioned along the heavily fortified border.

Pyongyang severed the communication line in March amid heightened tensions on the Korean Peninsula following the North's third nuclear test and its near-daily war threats. The two Koreas restored the western coastal hotline in September after they agreed to resume their joint industrial park in a North Korean border town of Kaesong.

Following the shocking execution of North Korea's unofficial No. 2, President Park Geun-hye ordered officials to study ways to revive the secretariat of the National Security Council to cope with the changing security situations on the peninsula. (Yonhap News)