The dramatic escalation of the simmering tensions between North Korea and South Korea has both U.S. and China expressing concern.

North Korea has set a 5 p.m. local deadline, or 4:30 a.m. EDT, to put a stop to anti-Pyongyang propaganda messages even as the two sides exchanged artillery fire across the Demilitarized Zone that separates the two countries. Kim Jong-Un has reportedly ordered his troops to “enter a wartime state to be fully battle ready to launch surprise operations.”

The tensions began when South Korea took up broadcasting propaganda messages from massive speakers on its side of the border after two of its soldiers were injured on August 4 by landmines believed to have been planted in the Demilitarized Zone by North Korea. Pyongyang has denied responsibility, and strongly objects to the broadcasts – which had once been routine but were discontinued more than 10 years ago. After the broadcasts resumed, North Korea fired artillery shells into South Korea on August. South Korea responded in kind but analysts say an attack from the North may be imminent if the loudspeakers were not switched of.