North Korea on Thursday threatened to make a "merciless" retaliation against South Korea, claiming that Seoul repeatedly violated its territorial waters and airspace off its western coast.

South Korea's reconnaissance drones made flights over the Yellow Sea along the de facto western maritime boundary on Tuesday and intruded three times into North Korea's territorial airspace to carry out aerial reconnaissance, the North's official Korean Central News Agency said.

It claimed the drones flew upwards of 10 kilometers into North Korean airspace.

The report also claimed that a South Korean Navy patrol boat and four fishing boats trespassed into the North's territorial waters five times on the same day.

"The escalated military provocation is sure to incite a merciless counteraction," the KCNA said in a statement.

Pyongyang does not acknowledge the western sea border, widely known as the Northern Limit Line, which was drawn unilaterally by the U.S.-led United Nations Command when the 1950-53 Korean War ended in a cease-fire.

The two Koreas had bloody confrontations in 1999, 2002 and 2009 along the NLL with the North shelling Yeonpyeong island near the demarcation line in late 2010. Several months before the shelling a North Korean submarine torpedoed a South Korean warship, killing 46 sailors. (Yonhap)