SEOUL (Reuters) - Amid signs that negotiations between North Korea and the U.S. are stalling, analysts say Pyongyang still sees its nuclear arsenal as a key tool in securing its national safety and winning concessions from international rivals.

A Digital Globe satellite image taken on March 29, 2018 shows what the Washington, D.C.-based Center for Strategic and International Studies (CSIS) Beyond Parallel project reports is an undeclared missile operating base at Sakkanmol, North Korea and provided to Reuters on November 12, 2018. CSIS/Beyond Parallel/DigitalGlobe 2018/Handout via REUTERS

Just as the United States has doubled down on its sanctions on Pyongyang, North Korean leader Kim Jong Un has not retreated from his pledge to expand his operational force of nuclear bombs and ballistic missiles, increasing his leverage under any still-elusive denuclearization deal.

A U.S. think tank said on Monday it had identified at least 13 of an estimated 20 active, undeclared missile bases inside North Korea, underscoring the challenge for American negotiators hoping to persuade Kim to give up his weapons programs.

As time goes by, North Korea’s likely expansion of its arsenal could force Washington to rethink its insistence on full denuclearization, said Moon Hong-sik, a research fellow at the Institute for National Security Strategy in Seoul.

“This is the choice the United States has to make: whether they keep pursuing the ideal of ‘complete, verifiable, irreversible denuclearization,’ or take this dilemma into consideration and make a compromise for limited denuclearization,” he said.

U.S. President Donald Trump met Kim at an unprecedented summit in Singapore in June where they agreed to “work toward complete denuclearization of the Korean Peninsula.

But with scant sign of progress on negotiations since and recent high-level meetings canceled, Trump said last week he’s now in “no rush” and still wants to meet with Kim for a second time.

U.S. officials have said sanctions forced North Korea to the negotiating table and vowed to keep pressure until complete denuclearization. But North Korea has credited its nuclear and missile breakthroughs for providing it the standing to meet the world’s largest powers.

Kim’s own words suggest Pyongyang will continue with production and development of the nuclear program even as it negotiates with Washington on denuclearization, experts say.

“In the 2018 New Year address, Kim Jong Un called for shifting to full-scale production and deployment of nuclear weapons and missiles,” said Joshua Pollack, a senior research associate at the U.S.-based James Martin Center for Nonproliferation Studies (CNS).

“He’s never said or done anything since then to contradict that.”

North Korea has not tested a nuclear bomb or ballistic missile since last year, and has said it has shuttered its main nuclear test site with plans to dismantle several more facilities.

North Korea recently warned, however, it could restart its nuclear program if the United States does not drop its campaign of “maximum pressure” and sanctions.

“NORTH KOREA NEVER PROMISED”

Monday’s report by the Washington, D.C.-based Center for Strategic and International Studies (CSIS), sparked media coverage calling it a “great deception” by the North Koreans.

But South Korea’s presidential Blue House said without an actual deal to violate, Pyongyang has broken no promises.

“North Korea has never promised to shut down this missile base,” Blue House spokesman Kim Eui-kyeom said. “It has never signed any agreement, any negotiation that makes shutting down missile bases mandatory... the fact that such a missile base exists shows the necessity for negotiations to be achieved quickly.”

Asked about the report, Trump’s national security adviser John Bolton told reporters in Singapore on Tuesday that Trump “has given North Korea an incredible opportunity to walk through a door to a different future if they denuclearize...but they still need to do that.”

The activity at the missile bases is one of several examples why American officials may be reluctant to lift any sanctions, said Shin Beom-chul, director of the Center for Security and Unification at Seoul’s Asan Institute for Policy Studies.

“In short, from the CSIS report we can infer that first, North Korea is not sincere enough with negotiating and second, there’s no change in their nuclear capacity,” he said.

U.S. officials have discussed possible clandestine enrichment sites for nuclear fuel, and in July, analysts at CNS used commercial satellite imagery to conclude that North Korea was “completing a major expansion of an important factory for producing solid rocket motors for... nuclear-armed missiles”.

In April, ahead of his meeting with Trump, Kim told a gathering of ruling party leadership the nuclear arsenal represented a “great victory” and announced there was no need for further tests of nuclear weapons or ballistic missiles.

In the speech, however, Kim also hinted at nuclear weapons playing an important role long into the future, calling them a “powerful treasured sword for defending peace” that would “reliably guarantee” a dignified and happy life for generations of North Koreans.

“Note the description of nuclear weapons as the only firm security assurance,” Pollack said. “They are not seeking any ‘security assurance’ other than the indefinite possession of nuclear weapons.”