President Trump welcomes South Korean President Moon Jae-in on June 30 at the White House in Washington. (Jim Bourg/Reuters)

President Trump spoke late Saturday to South Korean President Moon Jae-in amid increasing concerns in the White House that North Korea is not serious about striking a deal on denuclearization, which has complicated planning for the upcoming Singapore summit.

On the call, which lasted less than 30 minutes, Trump sought Moon’s interpretation of Pyongyang’s shift to a harder-line position last week, a sharp contrast to the more positive and constructive tone after Moon met with North Korean dictator Kim Jong Un last month, according to a senior U.S. official with knowledge of the conversation.

North Korea’s actions in recent days, including its cancellation of a working-level meeting with South Korean officials and a threat to call off Kim’s summit with Trump on June 12, have alarmed the Trump administration and created new complications in the preparations, with just over three weeks left. An advance team from the United States is in Singapore to work out logistics, Trump administration officials said.

National security adviser John Bolton has been telling colleagues that he doesn’t trust that the summit will go well, and he has reiterated his long-standing distrust of the North Koreans, a different person familiar with his views said.

Aides emphasized that Trump remains committed to meeting with Kim and that planning is moving forward but that time is running out to nail down an agenda and finalize several outstanding issues. The senior U.S. official said Pyongyang appears to be trying to extract more concessions from the United States before the summit, or to be building a narrative to blame Trump if things go poorly in Singapore, or to pull out of the summit entirely.

“People need to get real here after the euphoria of the peace summit” between Moon and Kim, said the U.S. official, who spoke on the condition of anonymity to discuss private deliberations. The official said the North Koreans have already failed to abide by some of their commitments in the “Panmunjom agreement” that was signed at the inter-Korea summit.

“They’re hedging, creating a sub-narrative,” the U.S. official said. “They’re not backing out yet. Neither is the president. It doesn’t look like they want to denuclearize at all.”

The official added: “It’s true there’s more coordination that needs to be done that hasn’t been done. There’s still time, but not a lot of it.”

Among the administration’s concerns is that although the Kim regime promised to destroy its nuclear test site, North Korea has not sanctioned any foreign inspectors or journalists to enter the country to confirm that such a process is underway.

Last week, a top Kim aide blasted Bolton, a North Korea hawk who has suggested the United States won’t lift economic sanctions until Pyongyang dismantles its nuclear program.

Many foreign policy and nuclear security experts said it remains highly unlikely that North Korea would be willing to abandon its program and that Kim’s goal is to establish himself on the world stage as the powerful leader of a nuclear-armed state.

“The North’s attitude is a pretty long distance away from what it appeared to be as Moon portrayed,” the U.S. official said. “It’s looking pretty different from that. It’s looking more like the old playbook.”

Moon is scheduled to visit Trump at the White House on Tuesday to coordinate strategy ahead of the Singapore summit. A liberal who took office last year, Moon has been an instrumental player in the diplomatic outreach to Pyongyang, having restarted long-dormant talks ahead of the Winter Olympics, which were held in South Korea.

That led to talks between the two countries, during which Kim offered to meet with Trump. The president accepted the invitation in March.

But Trump’s decision has been fraught with risk, given that former U.S. diplomats who have negotiated with North Korea under previous administrations have warned that the Kim family regime has a long history of violating international agreements to curb the nation’s nuclear weapons and ballistic missile programs.

Trump administration officials said they believe that Kim, after the remarkable images of him holding hands with Moon at the inter-Korea summit in late April, has returned to a harder-line position. Trump said last week that he believes the tone changed after Kim’s second visit to Beijing to meet with Chinese President Xi Jinping.

China has accounted for 90 percent of trade with North Korea, and Beijing’s cooperation in enacting U.N. Security Council sanctions on North Korea has been a crucial part of the Trump administration’s “maximum pressure” strategy.

“Since the China visit, they’ve moved pretty dramatically, shifted in the last several weeks to North Korea’s old position,” the U.S. official said.

Inside the West Wing, aides said Trump was upset by Kim’s first visit to China in March, ahead of which Beijing did not notify the White House. The president reacted angrily in a national security meeting, according to an administration official, and Xi later wrote a letter assuaging Trump.

Trump’s concerns about China’s influence on the summit with Kim were reflected in the president’s tweet last week in which he declared that his administration was looking at ways of potentially assisting ZTE, a Chinese phonemaker that has struggled in the wake of U.S. economic sanctions.

Trump’s public pronouncement was aimed, in part, at trying to keep the Chinese pressure on North Korea, said the administration official, who also spoke on the condition of anonymity to discuss internal deliberations.

On Thursday, Trump tried to reassure Kim that the United States would not seek his ouster in a denuclearization deal, a statement that appeared aimed at trying to keep the summit on track. But Trump also warned that if no deal was made North Korea could suffer the same fate as Libya, whose dictator, Moammar Gaddafi, was overthrown and killed in 2011 after that nation relinquished its nuclear program amid international pressure eight years earlier.

South Korea’s presidential Blue House said that during Moon and Trump’s phone call, they ­“exchanged views on North Korea’s recent statements.”

“The two leaders agreed to work closely together to ensure the June 12 U.S.-North Korea summit is a success,” said Yoon Young-chan, a spokesman for Moon.

Carol Morello, Josh Dawsey and Julie Tate in Washington and Anna Fifeld in Tokyo contributed to this report.