WASHINGTON — Defense Secretary Leon Panetta  is expressing relief that North Korea has not followed through on a threat to launch a military strike in response to South Korean activists floating anti-Pyongyang leaflets across the border.

Panetta told a news conference the United States and its ally South Korea would continue to watch for provocations from the North, and would be prepared to respond if they take place.

Panetta held talks at the Pentagon Wednesday with South Korean Defense Minister Kim Kwan-jin. The United States has nearly 30,000 troops on the southern side of the divided Korean Peninsula.

South Korean activists Monday floated balloons carrying tens of thousands leaflets critical of North Korean leader Kim Jong Un.

The activists eluded South Korean police trying to stop them.