In this Friday, Dec. 13, 2019, photo, an image showing North Korea's missile launch is displayed at the Unification Observation Post in Paju, South Korea, near the border with North Korea. North Korea on Saturday, Dec. 14, says it successfully performed another "crucial test" as its long-range rocket launch site that would further strengthen its "reliable strategic nuclear deterrent. (AP Photo/Ahn Young-joon)

In this Friday, Dec. 13, 2019, photo, an image showing North Korea's missile launch is displayed at the Unification Observation Post in Paju, South Korea, near the border with North Korea. North Korea on Saturday, Dec. 14, says it successfully performed another "crucial test" as its long-range rocket launch site that would further strengthen its "reliable strategic nuclear deterrent. (AP Photo/Ahn Young-joon)

SEOUL, South Korea (AP) — North Korea said it successfully performed another “crucial test” at its long-range rocket launch site that will further strengthen its nuclear deterrent.

The test — the second at the facility in a week, according to North Korea’s Academy of Defense Science — possibly involved technologies to improve intercontinental ballistic missiles that could potentially reach the continental United States.

Pak Jong Chon, chief of the Korean People’s Army’s general staff, asserted on Saturday that North Korea has built up “tremendous power” and that the findings from the recent tests would be used to develop new weapons to allow the country to “definitely and reliably” counter U.S. nuclear threats.

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The North in recent weeks has been dialing up pressure to coax major concessions from the Trump administration as it approaches an end-of-year deadline set by leader Kim Jong Un to salvage faltering nuclear negotiations.

The Academy of Defense Science did not specify what was tested on Friday. Just days earlier, the North said it conducted a “very important test” at the site on the country’s northwestern coast, prompting speculation that it involved a new engine for either an ICBM or a space launch vehicle.

The testing activity and defiant statements suggest that the North is preparing to do something to provoke the United States if Washington doesn’t back down and make concessions to ease sanctions and pressure on Pyongyang in deadlocked nuclear negotiations.

Stephen Biegun, the U.S. special representative for North Korea, was to arrive in Seoul on Sunday for discussions with South Korean President Moon Jae-in and other officials. It was unclear whether Biegun would attempt contact with North Korean officials at the inter-Korean border, which has often been used as a diplomatic venue, or whether such an effort would be successful.

“Genuine peace can be safeguarded and our development and future be guaranteed only when the balance of power is completely ensured. We have stored up a tremendous power,” Pak said in a statement carried by the North’s official Korean Central News Agency.

“We should be ready to cope with political and military provocations of the hostile forces and be familiar with both dialogue and confrontation,” Pak said. “Our army is fully ready to thoroughly carry out any decision of the supreme leader with action. ... U.S. and other hostile forces will spend the year-end in peace only when they hold off any words and deeds rattling us.”

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Kim Dong-yub, a former South Korean military officer and currently an analyst from Seoul’s Institute for Far Eastern Studies, said the North mentioning its nuclear deterrent makes it clear it tested a new engine for an ICBM, not a satellite-launch vehicle. Kim said it was notable that North Korea announced the specific length of the test, which he said possibly signals a more advanced design for a liquid-fuel multistage ICBM that can carry larger payloads with more stability.

North Korea’s current ICBMs, including the Hwasong-15, are built with first stages that are powered by a pair of engines that experts say are modeled after Russian designs. When the North first tested the engine in 2016, it said the test lasted for 200 seconds and demonstrated a thrust of 80 tons-force.

During a provocative run of weapons tests in 2017, Kim Jong Un conducted three flight tests of ICBMs that demonstrated potential range to reach deep into the U.S. mainland, raising tensions and triggering verbal warfare with President Donald Trump as they exchanged crude insults and threats of nuclear annihilation.

Experts say that the North would need further tests to establish the missile’s performance and reliability, such as improving its accuracy and ensuring that the warheads survive the harsh conditions of atmospheric reentry.

Relations between Kim and Trump became cozier in 2018 after Kim initiated diplomacy that led to their first summit in June that year in Singapore, where they issued a vague statement on a nuclear-free Korean Peninsula, without describing when or how it would occur.

But negotiations faltered after the United States rejected North Korean demands for broad sanctions relief in exchange for a partial surrender of the North’s nuclear capabilities at Kim’s second summit with Trump in Vietnam in February.

Trump and Kim met for a third time in June at the border between North and South Korea and agreed to resume talks. But an October working-level meeting in Sweden broke down over what the North Koreans described as the Americans’ “old stance and attitude.”

Kim, who unilaterally suspended nuclear and intercontinental ballistic missile tests last year during talks with Washington and Seoul, has said North Korea could seek a “new path” if the United States persists with sanctions and pressure against the North.

North Korea has also conducted 13 rounds of ballistic missile and rocket artillery tests since May, and has hinted at lifting its moratorium on nuclear and long-range missile tests if the Trump administration fails to make substantial concessions before the new year.

Some experts doubt that Kim would revive the tensions of 2017 by restarting nuclear and ICBM tests, which would cross a metaphorical “red line” and risk shattering his hard-won diplomacy with Washington. They say Kim is likely to pressure Trump with military activities that pose less of a direct threat to the U.S. and by bolstering a united front with Beijing and Moscow. Both are the North’s allies and have called for the U.N. Security Council to consider easing sanctions on Pyongyang to help nuclear negotiations move forward.