SEOUL, South Korea — North Korea's latest belligerent declaration — that it successfully tested a new long-range rocket engine that could allow nuclear strikes on the U.S. mainland — leaves outsiders in a familiar predicament.

With only a few details and no independent confirmation of what happened, there's the usual skepticism. But if even only part of the long list of nuclear and missile work that the North has boasted of successfully completing since its fourth nuclear test in January is true, Pyongyang would seem to be barreling ahead toward its goal of nuclear-armed long-range missiles.

The claim Saturday of a successful ground test of an intercontinental ballistic rocket engine, if true, would be another big step forward for young leader Kim Jong Un. But South Korean officials say North Korea doesn't yet have a reliable intercontinental ballistic missile, let alone the ability to arm it with a nuclear warhead.

The problem, as always, is that nothing has come close to checking North Korea's advance. International nuclear disarmament talks have been stalled for years, and round after round of tough U.N. sanctions have done little to halt the North's nuclear tests and long-range rocket launches, both of which are crucial to developing a nuclear missile arsenal.

The engine test, announced by the North's official Korean Central News Agency, follows last month's launch of a medium-range ballistic missile that violated U.N. Security Council resolutions that prohibit any ballistic activities by North Korea. It was the North's first medium-range missile launch since early 2014.

North Korea has also threatened pre-emptive nuclear strikes against Washington and Seoul and fired short-range missiles and artillery into the sea in an apparent response to ongoing U.S.-South Korean military drills and tough U.N. sanctions imposed over the recent nuclear test and a long-range rocket launch.

In Washington, State Department spokesman Mark Toner called on North Korea to "refrain from actions and rhetoric that further destabilize the region and focus instead on taking concrete steps toward fulfilling its commitments and international obligations." There was no immediate comment by the South Korean government over the North's announcement.

Some analysts think Kim's belligerent stance is linked to a major ruling party congress next month meant to further cement his grip on power. The outside pressure and anger caused by bombastic threats and repeated nuclear-related tests, the argument goes, is meant to rally the North Korean people around Kim as he stands up to powerful enemies that Pyongyang says are trying to crush the North.

× Fear of missing out? Sign up for the Early Bird Brief - a daily roundup of military and defense news stories from around the globe. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

It is also possible that efforts to promote military accomplishments to a domestic audience are meant to make up for a lack of tangible economic achievements ahead of the Workers' Party congress, the first since 1980, said Kim Dong-yub, a North Korean expert at Seoul's Institute for Far Eastern Studies.

With typical rhetorical flourish, the North's KCNA said that Kim was delighted as the "high-power" rocket engine spewed out "huge flames with (a) deafening boom" during the ground test at the Sohae Space Center in the country's northwest, the site of its February long-range rocket launch. KCNA did not say when the test was conducted.

The news agency quoted Kim as saying that the North can now tip intercontinental ballistic missiles with more powerful nuclear warheads that could keep the U.S. mainland within striking distance and "reduce them to ashes so that they may not survive in our planet."

The North's official Rodong Sinmun newspaper published photos of Kim smiling widely and clapping with officials. A burst of flames can be seen coming out of an object strapped to what appears to be an outdoor vertical tower structure. The veracity of the photographs could not be independently confirmed.

While the newspaper's photos provided only limited information of what North Korea had done, they do indicate that the country is trying to create a different long-range rocket engine from the ones it has used on its space launch vehicles, said Lee Choon Geun, an analyst at South Korea's state-funded Science and Technology Policy Institute.

He pointed to differences in the rocket engines' shape and size and the way flames come out of them. It's impossible to tell how successful the North's recent test was without detailed data, Lee said.

Experts say a militarized version of the rocket the North used to put its second satellite into orbit in February would potentially have the range to reach the U.S. mainland. However, North Korea's possible candidates for an intercontinental ballistic missile have never been tested "end-to-end," from launch through re-entry and warhead delivery, to show they actually work.

The North has gone to great lengths in recent months to tout alleged advancements in its nuclear weapons and ballistic missile programs. Those claims have often been met with doubt by South Korean officials and experts.

The North's official media on March 9 showed a smiling Kim posing with nuclear scientists beside what appeared to be a model trigger device of a nuclear warhead. Kim declared that warheads had been miniaturized for use on ballistic missiles.

The North has also claimed to have mastered a re-entry technology designed to protect a warhead from extreme heat and other challenges when it returns to the atmosphere from space following a missile launch. It also said it had successfully conducted a high-powered, solid-fuel rocket engine test. Solid-fuel missiles are generally harder to detect before they are launched than liquid-fuel missiles.

Copyright 2016 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.