Updated

North Korea has threatened to attack South Korea "without notice" in response to demonstrations held in the South to mark the anniversary of its former leader's death.

South Korea’s Defense Ministry said a letter from the North’s National Defense Commission was faxed early Thursday via a military communication link between the two sides, warning of a "merciless" attack on the South, The Wall Street Journal reported Friday.

The letter objected to the "repeated extra-large provocations to North Korea's highest dignity taking place in the middle of Seoul" and threatened "a merciless retaliation without warning," ministry spokesman Kim Min-seok told the newspaper.

The warning came after conservative activists and North Korean refugee groups held demonstrations in Seoul on the anniversary of Kim Jong Il's death. Some protesters reportedly burned pictures of current leader Kim Jong Un, according to The Telegraph.

The South Korean government reportedly responded to Pyonghang's threats by sending a fax to that promised "resolute punishment" in response to any provocation from the North.

North Korea has recently turned to attempts at diplomacy with South Korea and the United States. But tensions have remained high since Pyongyang's threats in March and April, which included warnings that it would restart nuclear bomb fuel production.

Former NBA star Dennis Rodman held tryouts Friday for a North Korean team to face a dozen NBA veterans in an exhibition game on Kim's birthday next month.

The flamboyant Hall of Famer said plans for the Jan. 8 game are moving ahead but some of the 12 Americans he wants are afraid to come to Pyongyang.

The Associated Press contributed to this report.

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