North Korean leader Kim Jong Un's latest harsh rhetoric toward the United States underscores his judgment that it will be difficult to gain concessions from the new administration of President-elect Joe Biden any time soon, diplomatic sources said.

North Korean leader Kim Jong Un is pictured at the congress of the Workers' Party in Pyongyang. (KCNA/Kyodo)

While placing priority on bolstering the country's nuclear capacity, Pyongyang is likely to prepare for a long battle against Washington in a bid to attain Kim's cherished goal of ensuring U.S. security guarantees to his rule, the sources said.

At the first congress of the ruling Workers' Party in nearly five years, Kim clearly showed a confrontational attitude toward the United States, describing Washington as Pyongyang's "biggest enemy," state-run media reported Saturday.

For the past few years, North Korea has avoided excessive provocations, such as test-firing an intercontinental ballistic missile that could target the U.S. mainland, as Kim has tried to build a better rapport with Republican President Donald Trump.

But fears are growing that after Biden, who is expected to assume a tougher stance against North Korea, takes office on Jan. 20, U.S.-North Korea negotiations on denuclearization may remain stalled and eventually end up back at square one.

In the worst-case scenario, Pyongyang might resume nuclear tests and launches of ICBMs, which would make the security environment in East Asia more uncertain and even fragile, the sources said.

Biden will "not make an easy deal with Kim Jong Un," given that the president-elect favors bottom-up policymaking, unlike Trump, who pursues a top-down approach that would help set the mood for his "political show," one of the sources said.

"The Biden administration may take a long time to map out its policy toward North Korea. In the interval, Kim would attempt to strengthen the nation's military capacities to survive a prolonged battle with the United States," the source said.

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"It is very dangerous to give North Korea time to develop new weapons," he added.

The United States and North Korea are still technically in a state of war as the 1950-1953 Korean War ended in a cease-fire rather than a peace treaty.

During the war, U.S.-led U.N. forces fought alongside South Korea against the North, backed by China and the Soviet Union. Since then, the United States and North Korea have not had diplomatic ties.

Under the administration of former U.S. President Barack Obama, in which Biden served as vice president for eight years from 2009, Washington adopted a "strategic patience" policy, designed to put more pressure on Pyongyang while waiting for it to return to talks.

In November 2017, North Korea fired what it said was its "most powerful" ICBM capable of delivering a nuclear warhead anywhere in the continental United States. Afterward, Kim declared the completion of "the state nuclear force."

Since 2018, Kim suddenly extended an olive branch to Trump, who became U.S. President in 2017. The two leaders have held direct meetings three times, with North Korea refraining from carrying out nuclear and ICBM tests.

At their first-ever summit in June 2018 in Singapore, Trump and Kim agreed Washington would provide security guarantees to Pyongyang in exchange for "complete" denuclearization of the Korean Peninsula.

The following year, they fell short of bridging the gap between demands by the United States and North Korea's calls for sanctions relief at their second summit in the Vietnamese capital Hanoi.

After Trump and Kim surprisingly met in July 2019 at the Demilitarized Zone dividing the two Koreas, Washington and Pyongyang held a working-level meeting in Stockholm in October that year, but it ended without progress.

Kim, however, has strived to maintain personal relations with Trump by exchanging letters to promote negotiations with the U.S. president over security guarantees and sanctions relief.

So far, Biden has criticized the consequences of the Trump diplomacy for allowing North Korea to have "much more capable missiles, able to reach U.S. territory much more easily than they ever did before," and calling Kim a "thug."

Another diplomatic source in Beijing said, "Such remarks might have prodded Kim to take a hardline stance against Biden."

At the ruling party congress, meanwhile, Kim said that the United States would have to abandon its "hostile" policy toward North Korea to establish a "new relationship" between the two countries.

He also said, "Strong defense capabilities of the state never preclude diplomacy but serve as a great means that propels toward the correct orientation and guarantees its success."

The source in Beijing said, "North Korea still has intentions to continue talks with the United States, so Biden should leave room for dialogue with Kim for peace and stability in East Asia."

At home, North Korea's economy has languished after cutting off traffic to and from its neighbors, China and Russia, since early last year to prevent the novel coronavirus, first detected in the Chinese city of Wuhan, from entering the nation.

Kim has acknowledged that North Korea failed to achieve the economic development goals set in its strategy through 2020 in almost all sectors.

"North Korea may be keen to talk about economic sanctions relief with the United States. In order to stop Kim from increasing security threats in the region, Biden should craft his policy toward North Korea as soon as possible," the source said.