Leader Kim Jong Un guided Thursday’s test, and warned it would make enemies “experience a clearer security crisis, and constantly strike extreme uneasiness and horror into them by taking fatal and offensive counter-actions until they abandon their senseless thinking and reckless acts”, North Korean state media said.

Analysts said it was the North’s first use of solid propellants in an intermediate-range or intercontinental ballistic missile, a key task to deploying missiles faster during a war.

South Korea’s defence ministry said North Korea was still developing the weapon, and that it needed more time and effort to master the technology, indicating that Pyongyang might carry out more tests.

North Korean state media outlet KCNA released photos of Kim watching the launch, accompanied by his wife, sister and daughter, and the missile covered in camouflage nets on a mobile launcher. A state media video showed the Hwasong-18 missile blasting off from a launch tube, creating a cloud of smoke.

The development of the Hwasong-18 will “extensively reform the strategic deterrence components of the DPRK, radically promote the effectiveness of its nuclear counterattack posture and bring about a change in the practicality of its offensive military strategy,” KCNA said, using the initials of the country’s official name.

South Korea and the U.S. air forces staged drills hours after the report, involving American B-52H bombers that joined F-35A, F-15 and F-16 fighter jets, Seoul’s defence ministry said.

“By deploying U.S. strategic assets with increased frequency and intensity, the two countries will continue demonstrating our strong alliance’s will that we will never tolerate any nuclear attack from North Korea,” the ministry said in a statement.

North Korea has criticised recent U.S.-South Korean joint military exercises as escalating tensions, and has stepped up weapons tests in the past months.

Japan also conducted separate air drills with two U.S. B-52 bomber jets on Friday, accompanied by four U.S. F-35 fighters and four Japanese F-15 fighters, Tokyo’s defence ministry said. It marked a second consecutive day of a Japan-U.S. joint air mission over the Sea of Japan.

Japan asked the United Nations Security Council to convene an emergency meeting on North Korea’s ballistic missile launches, top government spokesperson Hirokazu Matsuno told a Friday press conference.

MORE TESTS?

Most of North Korea’s largest ballistic missiles use liquid fuel, which requires them to be loaded with propellant at their launch site – a time-consuming and dangerous process.

“For any country that operates large-scale, missile based nuclear forces, solid-propellant missiles are an incredibly desirable capability because they don’t need to be fuelled immediately prior to use,” said Ankit Panda, a senior fellow at the U.S.-based Carnegie Endowment for International Peace. “These capabilities are much more responsive in a time of crisis.”

North Korea will most likely keep some liquid-fuel systems, complicating the calculations of the U.S. and its allies during a conflict, Panda said.

Vann Van Diepen, a former U.S. government weapons expert who now works with the 38 North project, said solid-fuel missiles are easier and safer to operate, and require less logistical support – making them harder to detect and more survivable than liquids.

North Korea first displayed what could be a new solid-fuel ICBM during a military parade in February after testing a high-thrust solid-fuel engine in December.

Analysts said the U.S. could determine between a solid- or liquid-fuelled launch with early warning satellites that can detect differences in the infrared data produced by various missile types.

The latest launch came days after Kim called for strengthening war deterrence in a “more practical and offensive” manner to counter what North Korea called moves of aggression by the United States.

The missile, fired from near Pyongyang, flew about 1,000 km (620 miles) before landing in waters east of North Korea, officials said. North Korea said the test posed no threats to its neighbouring countries.

A South Korean military official said the missile’s maximum altitude was lower than 6,000 km, the apogee of some of last year’s record-breaking tests.

“North Korea could have opted to focus on collecting data necessary to check its features at different stages than going full speed at the first launch,” said Kim Dong-yup, a professor at the University of North Korean Studies. “As it was a test that did not demonstrate its normal flight pattern, North Korea will likely conduct some more tests.”

By Soo-hyang Choi and Ju-min Park

(Reporting by Soo-hyang Choi and Ju-min Park in Seoul, David Brunnstrom in Washington and Kantaro Komiya in Tokyo; Additional reporting by Hyonhee Shin; Editing by Leslie Adler, Sandra Maler, Sonali Paul, Gerry Doyle and Raju Gopalakrishnan)