North Korea's front-line troops staged a river-crossing attack exercise, state media said Tuesday, as Pyongyang is upset about U.S. President Barack Obama's recent remarks and the continued scattering of propaganda leaflets by South Korean activists.

North Korean leader Kim Jong-un "organized and guided" the winter drill of the armored infantry sub-units of the motorized strike group in the western sector of the front" of the country's military, reported the Korean Central News Agency (KCNA). It did not specify the timing and venue.

Kim was quoted as stressing the importance of honing capabilities to mount a "deadly attack," saying a "powerful offensive means rock-firm defense."

A set of photos released by the KCNA showed Kim watching the training and a couple of top military officials -- Hwang Pyong-so and Hyon Yong-chol -- directly leading it aboard a mobile artillery launcher and an armored vehicle. Both Hwang, director of the military's general political bureau, and Hyon, minister of defense, are 66 years old.

Carrying a similar report, the (North) Korean Central Broadcasting Station described the U.S. as a deadly foe and claimed it blatantly revealed an ambition to topple the socialist system.

Last week, Obama said in an interview on YouTube that the isolated communist regime is likely to collapse mainly due to the Internet-driven inflow of information.

The North's broadcaster also condemned the latest spread of anti-Pyongyang leaflets via balloons in the country by a group of conservative activists in the South.

On Sunday, the National Defense Commission, Pyongyang's top ruling organ, warned of stern punishment against South Korea, accusing it of allowing the leaflet campaign and planning to start annual joint military drills with the U.S. in March. (Yonhap)