SEOUL, South Korea — North Korea said on Thursday that it had successfully tested a new rocket engine that would significantly bolster its missile capabilities, and South Korea called on its people to heighten vigilance against possible terrorist attacks by the North.

The North’s leader, Kim Jong-un, recently watched as military engineers conducted a ground test of the design, a solid-fuel rocket engine, the state-run Korean Central News Agency reported.

Except for some of its short-range missiles and rockets, most of the North’s missiles, including the long-range booster rocket used to launch a satellite into orbit last month, are believed to rely on liquid fuel. The use of solid fuel greatly shortens the time needed to prepare the launch of a missile as well as increases the ground mobility of the missile system, which would reduce the South’s ability to detect signs of a pending attack, South Korean defense officials said.

“The North’s switching to solid rocket fuel means that it wants to have a capability to launch a missile anytime it wants,” said Moon Sang-gyun, a spokesman for the South’s Defense Ministry, calling the development “grave.”