South Korea's National Security Council on Thursday received a fax from the North Korea's National Defence Commission in which Pyongyang threatened to strike the South Korea "without any notice", a news agency reported.

The hostile rhetoric came after some conservative groups on Tuesday held rallies in Seoul to protest against Kim Jong-un's regime for the execution of Jang Song Thaek. The North Korea military condemned the rallies that insulted the country's "highest dignity" and vowed to retaliate against the South Korea.

The South Korean Defence Ministry responded, pledging to "sternly react" to any provocation, according to the ministry spokesman Kim Min-seok. "We are closely monitoring the North Korean military's move, preparing to sternly react to any provocation," Yonhap news agency quoted Kim as saying.

Kim added that there was no indication of unusual activity by North Korea's military so far as they are carrying out its routine winter drills since early December.

South Korean Defence Minister Kim Kwan-jin said that North Korea may conduct provocations against South Korea in early 2014, noting that internal disorder within North Korea can be escalated following the execution of Jang Song-Thaek, uncle of leader Kim Jong Un.

(Agencies)

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