North Korea might be preparing for another nuclear test, satellite images suggest

North Korea appears to be getting ready for another nuclear test, according to new satellite images that show a prolonged and heightened level of activity at its underground testing site.

It's impossible to tell if North Korea is putting on a performance for the satellites - a ploy it has sometimes used to raise tensions - or if a sixth nuclear test is imminent. But analysts agree that North Korea is determined to make progress on its nuclear and missile programs.

"They are trying to get a working arsenal, so the more they test, the more they learn," said Jon Wolfsthal, a senior nonproliferation adviser in the Obama administration who is now at the Carnegie Endowment for International Peace.

"It's likely that they're trying to make a device small enough to achieve their goal of putting a nuclear warhead on a long-range missile," Wolfsthal said.

North Korea has made no secret of its ambitions to build a missile capable of reaching the continental United States, and Kim Jong Un said in a Jan. 1 address that his regime had "entered the final stage of preparation for a test-launch of an intercontinental ballistic missile."

It has been launching medium- and intermediate-range missiles and testing rocket engines at a surprisingly fast pace over the past year, tests that analysts view as part of a bigger ICBM program.

Pyongyang has also bragged about its progress with nuclear weapons, claiming following its last test, in September last year, that it had successfully miniaturized a warhead. At the time, a state newsreader warned the United States that North Korea was able to "counterattack" if its enemies invaded.

With the Trump administration warning that "all options are on the table" to stop the North Korean threat, and South Korea and the United States conducting joint military exercises on the southern half of the Korean Peninsula, tensions are running high in the region.

North Korea's foreign ministry issued another angry diatribe against the exercises Thursday, saying that it would "mount a resolute preemptive attack" if it thought the United States was about to strike it.

"In case a war breaks out on the Korean peninsula, the U.S. will be held wholly accountable for it, no matter who will launch a preemptive attack, as it is causing trouble by bringing lots of nuclear strategic assets and special warfare means," said the statement, carried by the official Korean Central News Agency.

Added to these tensions, April is an important month on the North Korean calendar - the regime celebrates the anniversary of founding president Kim Il Sung's birthday on April 15 - and North Korea has a habit of timing provocative actions with key dates.

The next provocative action could be a nuclear test.

New commercial satellite imagery of the Punggye-ri nuclear test site in the northeastern part of North Korea shows an increased level of activity there.

"It certainly looks like they're preparing for another test," Joseph S. Bermudez Jr., chief analytics officer for AllSource Analysis, a Denver-based consultancy, said after reviewing new satellite images. The images were published on the 38 North specialist website Wednesday.

Photos taken Tuesday show a group as many as 100 people standing outside the main administration building at the test site.

"The last time we saw a large formation of people like this was six weeks before the February 2016 test," Bermudez said. "When you look at the way they're draining water and removing rubble, it sure looks like they're getting ready for another test."

Images taken between March 24 and 28 show four or five vehicles or trailers at the entrance to the north portal of the underground site and machinery to pump water out of the site, presumably to keep the tunnels dry for communications and monitoring equipment that appear to have been installed.

"The North Koreans know when commercial satellites are passing overhead and typically try to avoid activities during that time. The fact these formations can be seen suggests that Pyongyang is sending a political message that the sixth nuclear test will be conducted soon," Bermudez and fellow analyst Jack Liu wrote for 38 North.

"Alternatively, it may be engaged in a well-planned game of brinkmanship," they wrote.

South Korean government officials also see signs of preparations for another test.

"We have intelligence that North Korea may conduct its sixth nuclear test in the first week of April and are in the process of confirming this," an unnamed South Korean military intelligence official told the JoongAng Ilbo newspaper. "We have determined that North Korea is expected to finish all preparations for a nuclear test by March 31."

Furthermore, South Korean officials say they have spotted Kim's private airplane at an airfield near the Punggye-ri test site.

"Our assessment is that the North is ready to conduct the nuclear test when they get the green light from Kim Jong Un," Lee Duck-haeng, a spokesman for the unification ministry, which is in charge of the South's relations with the North, told reporters Wednesday.

But Joshua Pollack, editor of the Nonproliferation Review, was skeptical of the images, saying that they showed an active site, "as they always do."

"Many people are anticipating a test in light of the recent North Korean missile tests and a reading of the diplomatic tea leaves," Pollack said. "The picture probably shouldn't lead anyone to change that judgment one way or the other."

While April 15 is an auspicious day in North Korea and the Supreme People's Assembly, its rubber-stamp parliament, will be convened on April 11, Kim might see an opportunity to interrupt another bilateral summit in the United States.

North Korea tested an intermediate-range ballistic missile, its first launch since Donald Trump was elected president, in February just as Trump was sitting down for dinner at his Mar-a-Lago resort in Florida with Japanese Prime Minister Shinzo Abe.

Trump is expected to host Chinese President Xi Jinping at the resort on April 6 and 7.

Regardless of the timing, many analysts expect another nuclear test, after two were conducted last year.

"Our sense has been for more than a few months that they could test at any time," said Wolfsthal of Carnegie. "History suggests that they want to be prepared to test because we've started military exercises, and because there is a new president in the United States and they want to show him they are tough and won't back down."

North Korea has conducted five nuclear tests, three of them since Kim Jong Un took over the country after the death of his father at the end of 2011. Four of them took place inside the north portal of the test site at Punggye-ri.