SEOUL (Reuters) - North Korea successfully tested a solid-fuel engine that boosted the power of its ballistic rockets, state media reported on Thursday, as South Korea’s president ordered the military to be ready to respond to the North’s “reckless provocation.”

North Korean leader Kim Jong Un visits the February 11 plant at the Ryongsong Machine Complex in this undated photo released March 24, 2016. REUTERS/KCNA

Pyongyang’s claim indicates it is continuing to develop an intercontinental ballistic missile (ICBM) at a rapid pace in defiance of U.N. sanctions, and amid assessment by the South’s officials that it could conduct a new nuclear test at any time.

The isolated state has in recent weeks stepped up bellicose rhetoric, threatening pre-emptive nuclear strikes against Washington and Seoul, as well as making claims of advancement in its weapons technology.

The Rodong Sinmum, North Korea’s ruling party newspaper, carried photos of leader Kim Jong Un on site as a rocket engine laid horizontally on the ground emitted a fiery blast. A two-page report detailed the testing of the engine’s structure and thrust.

“He noted with great pleasure that the successful test ... helped boost the power of ballistic rockets capable of mercilessly striking hostile forces,” KCNA news agency said.

North Korea said last week it had conducted a successful simulated test of atmospheric re-entry of a ballistic missile, and would soon test ballistic missiles capable of carrying nuclear warheads.

On Wednesday, North Korea repeated a threat to attack the South’s presidential office, saying its large-calibre multiple rocket launch systems are on alert to strike the Blue House and its special operations unit is ready to go into action.

South Korean President Park Geun-hye office said she had ordered a heightened state of alert and put the military on standby to “respond actively to reckless provocations by the North.”

U.S. State Department spokesman Mark Toner repeated a call on North Korea to “refrain from any actions and any rhetoric that raise tensions in the region and comply with its international obligations and commitments.”

The current tension on the peninsula follows tough new U.N. sanctions against the North over its nuclear and missile programs and coincides with annual military drills by the South Korea and the United States.

The North calls the exercises “nuclear war moves” and has threatened to respond with an all-out offensive. It has conducted a series of rocket launches in recent days.

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SOLID FUEL ROCKET ENGINE

Pyongyang has previously launched long-range rockets that used liquid fuel but it was seen to lack the capability to build solid-fuel long-range or intercontinental missiles.

Solid-fuel rockets have advantages in military use, although liquid fuel rockets are considered more sophisticated as their thrust can be controlled in flight.

The North has deployed short and medium-range missiles and test fired them, but never flight-tested the KN-08 ICBM it is believed to be developing.

Despite its boasts to be making progress, many experts believe the North is a decade or more away from building an ICBM capable of threatening the United States.

Michael Elleman, a U.S.-based expert on solid rocket fuel with the International Institute for Strategic Studies think tank, said the engine North Korea tested appeared to have been for the upper stage of a larger rocket or missile.

He told a Washington seminar hosted by the Washington-based North Korean monitoring project 38 North that Pyongyang was at least 15 years away from being able to produce solid-fuel motor large enough to able power an ICBM.

He said it was unlikely North Korea was producing rocket engines itself and was probably instead relying on a stockpile from the Soviet era.

Elleman said he found it "shocking" that Kim Jong Un was photographed standing a few yards away from the rocket motor, apparently just before the test. (www.rodong.rep.kp/en/) He said there was a risk of an inadvertent explosion from "one, maybe one-and-a-half, possibly even two tonnes of propellant."

“Solid rocket propellant is essentially an explosive that burns at slower rate,” he said. “They were putting Kim Jong Un at risk by having him near such a volatile system.”

North Korea’s stepped-up rhetoric and weapons claims come ahead of its planned congress of the ruling Workers’ Party, the first in more than 35 years.

Some Pyongyang-watchers say the North may look to claim a splashy achievement, such as a fifth nuclear test, in the run-up to the congress as young leader Kim Jong Un looks to bolster his domestic legitimacy.

North Korea is ready to conduct a fifth nuclear test “now, immediately,” South Korea’s unification ministry said on Monday.