North Korea on Monday ordered its troops onto a state of full combat readiness as the United States and South Korea started military drills, Yonhap reported, citing the North''s official KCNA agency.

The South Korean news agency said an English language KCNA report on the military order warned of a "merciless" response to any aggression that could include the country''s "nuclear deterrent."

Seoul and Washington say the exercises are purely defensive.

The latest rhetoric over the military drills, which followed a threat on Sunday to "wipe from the face of the earth" the United States and South Korea, comes amid a reported thaw in cultural ties between the two Koreas.

North Korea and South Korea''s Hyundai Group on Monday agreed to resume tourist trips to Mount Kumgang in the northeast of the peninsula and the North''s border town of Kaesong, Yonhap reported.

"Both sides decided to resume the suspended tours to Mount Kumgang as soon as possible and launch the tour of Pirobong, the highest peak on the mountain," said a joint press release issued by Hyundai and the North''s Korea Asia-Pacific Peace Committee, which handles inter-Korean business ties.