After North Korean leader Kim Jong Un declared a "quasi state of war" for the nation's troops near the South Korean border, Tokyo expressed "strong concerns" on Friday and condemned Pyongyang for its earlier "provocation" of artillery strikes across the border.

"The government is deeply concerned over the action (by the North). We urge North Korea to refrain from carrying out such acts of provocation," Chief Cabinet Secretary Yoshihide Suga told a regular news conference Friday.

On Thursday, the North reportedly fired shells in the direction of one of South Korea's propaganda loudspeaker units. In retaliation, the South fired artillery shells back. No casualties were reported.