North Korea’s launch of two “tactical guided missiles” Friday were part of a “short-notice” drill of the nuclear-armed country’s new railway-borne missile regiment, state-run media said Saturday.

The North’s third and fourth launches over the last 10 days, came from Uiju in North Pyongan province, which borders China, and flew around 430 km (270 miles) at an altitude of 36 km and hit a top speed of Mach 6, or six times the speed of sound, according to the South Korean military.

Friday’s launches came hours after Pyongyang vowed a “stronger and certain reaction” to U.S. sanctions slapped on five North Koreans over the country’s nuclear and missile programs.