North Korean leader Kim Jong Un has ordered the country's key science institute to produce more solid-fuel rocket engines and warhead tips, state media reported Wednesday.

(Korea Media/Kyodo)

Kim was briefed about the process to manufacture intercontinental ballistic missile warhead tips and solid-fuel rocket engines during his inspection of the Chemical Material Institute of the Academy of Defense Science, the Korean Central News Agency said without mentioning when the visit was made.

"He instructed the institute to produce more solid-fuel rocket engines and rocket warhead tips by further expanding the engine production process and the production capacity of rocket warhead tips and engine jets by carbon and carbon compound material," KCNA said.

The institute produces intercontinental ballistic missile warheads and also develops carbon compound materials used in rocket engines.

It was the first instance of official media reporting so-called field guidance from Kim since Aug. 15 when his remarks indicated North Korea would hold off on its threat to fire missiles into waters near the U.S. territory of Guam.

KCNA said Kim "gave special thanks and a special bonus" to officials at the institute, describing them as "unassuming heroes" and "patriotic scientists."

Kim's order to increase production of solid-fuel rocket engines and warhead tips is seen as reflecting Pyongyang's resolve, despite international condemnation, to develop intercontinental ballistic missiles capable of reaching the U.S. mainland.

Solid-fuel engines help cut the required preparation time for a missile launch and make it more difficult for outside observers to detect a launch in advance. For example, the Pukguksong-2 missile, a new medium- to long-range ballistic missile tested by the North, is known to be using pre-loaded solid fuel.

Meanwhile, North Korea's official newspaper Rodong Sinmun released photos that appears to show the country may be developing a new submarine-launched, solid-fuel ballistic missile.

On a wall at the institute inspected by Kim was a sign board showing a concept diagram for the underwater strategic Pukguksong-3 missile.

It also showed Kim picking up an object that could be the tip of a missile warhead, which appeared to be intended to counter skepticism of experts in the United States, South Korea and Japan as to whether North Korea has mastered the technology needed to ensure the survival of an ICBM-mounted nuclear warhead when re-entering the Earth's atmosphere from space.

Despite the start of a 10-day joint military exercise by U.S. and South Korean troops this week, Wednesday's KCNA report did not include any provocative rhetoric against the United States nor remarks regarding a plan to launch another missile.

In the United States, President Donald Trump expressed cautious hope on Tuesday for possible improvement in relations with North Korea, saying at a campaign rally in Phoenix, Arizona, that Kim "is starting to respect us."

"And maybe, probably not, but maybe something positive can come about," Trump said.

Prior to Trump's speech, U.S. Secretary of State Rex Tillerson suggested in Washington the same day that dialogue with North Korea may be possible in the near future, as he hailed the restraint Pyongyang has shown recently with its weapons programs.

"I think it is worth noting that we have had no missile launches or provocative acts on the part of North Korea since the unanimous adoption of the U.N. Security Council resolution" early this month that toughened sanctions on Pyongyang, he told reporters at the State Department.

North Korea successfully tested two intercontinental ballistic missiles last month, including one on July 28 that experts said puts U.S. mainland cities -- such as Los Angeles, Denver and Chicago -- within range if fired at a normal trajectory.