SEOUL, South Korea — North Korea’s state news media released a photograph Wednesday suggesting that the North was working on a more powerful solid-fuel ballistic missile, and said the country’s leader, Kim Jong-un, had ordered the production of more rocket engines and warheads.

Unlike liquid-fuel rockets, solid-fuel missiles do not have to be loaded with fuel just before launching, a process that can take up to an hour and make the missile vulnerable to a pre-emptive strike. Such missiles are also easier to transport and hide.

Mr. Kim gave his order to bolster the country’s missile arsenal during a visit to the Chemical Material Institute of the Academy of Defense Science, the North’s official Korean Central News Agency said, without disclosing when the visit took place.

“He instructed the institute to produce more solid-fuel rocket engines and rocket warhead tips by further expanding engine production process and the production capacity of rocket warhead tips and engine jets,” the news agency said of the visit.