Kim Jong Un urged "positive and offensive measures" to bolster North Korea's security, as the Trump administration said it was watching for provocations around the regime's year-end deadline.

»PHOTOS: Trump meets with North Korea's Kim Jong Un in Vietnam

Kim issued his call for action during an unusually large and lengthy meeting of North Korea’s ruling Workers’ Party, which continued behind closed doors Monday in Pyongyang. Donald Trump’s national security adviser, Robert O’Brien, who credited the president with persuading Kim to forgo a destabilizing action during the Christmas holiday, said the administration was monitoring the situation.

“President Trump took a different tack with personal diplomacy, and so far we’ve had some success,” O'Brien said Sunday on ABC’s “This Week” program. “So perhaps he’s reconsidered that, but we’ll have to wait and see.”

The historic summit took place in Singapore.

While Christmas came and went without the “gift” North Korea had threatened to deliver to Trump for the holiday, all signals from Pyongyang in recent months have pointed to escalation in the new year. Kim has vowed to take a “new path” in nuclear talks without further U.S. concessions, and North Korea promised to issue its “final judgment” on Trump by the end of this month.

North Korean state media has so far said little about the ongoing Workers’ Party meeting, although the size and duration of the gathering suggested it was among the most significant since Kim took power in 2011. The North Korean leader was expected to deliver a New Year’s address Wednesday, which was being watched even more closely than usual for signs of whether he plans to escalate tensions in the coming months.

9. On January 3, China landed the first spacecraft on the so-called dark side of the moon. 8. On March 10, an Ethiopian jetliner carrying 157 people crashed, killing everyone aboard. 7. On March 15, 50 people were shot to death in the New Zealand city of Christchurch in a terror attack at two mosques during midday. 6. On April 15, a fire nearlydestroyed the Notre DameCathedral in Paris. 5. On May 6, Prince Harry and Duchess Meghan announced the birth of their baby Archie Harrison Mountbatten-Windsor.

During the second day of the party meeting Sunday, Kim made remarks “emphasizing the need to take positive and offensive measures for fully ensuring the sovereignty and security of the country as required by the present situation,” the state-run Korean Central News Agency said without elaborating. A spokesman for South Korea’s Unification Ministry said the meeting was the largest party gathering held since at least 2013.

“It could be interpreted as North Korea considering 2020 as a very significant year in terms of managing the nation as well as economy construction,” said Kim Dong-yub, a professor of North Korean studies at Kyungnam University and a former director of nuclear weapons policy at South Korea’s defense ministry. “It could also indicate that the party is considering this year to be a crucial make-or-break point for Kim Jong Un’s leadership and the stability of it.”

VIDEO: Trump, Kim Jong Un Sign Agreement on Denuclearization

Kim has suspended tests of nuclear weapons and intercontinental ballistic missiles that could deliver a warhead to the U.S. since he began talks with Trump in 2018. But he has also been busy expanding his nuclear arsenal in that time —rolling out a new line of solid-propellant, nuclear-capable ballistic missiles and producing enough fissile material for about a dozen or so more nuclear bombs, weapons experts have said.

O’Brien, 53, a former hostage envoy and Trump’s fourth national security adviser, said the U.S. was ready to respond should Kim fire additional long-range missiles or conduct further nuclear weapons tests. The Pentagon sent a guided-missile destroyer, the USS Milius, to waters between the Korean Peninsula and Japan, the Wall Street Journal reported, citing anonymous U.S. defense officials.

“We'll reserve judgment, but the United States will take action as we do in these situations,” O'Brien said, incorrectly referring to Kim as “Chairman Un” before switching to his full name. “If Kim Jong Un takes that approach, we’ll be extraordinarily disappointed and we’ll demonstrate that disappointment.”

Trump has touted his outreach to Pyongyang, and his personal ties to Kim, as a foreign policy triumph. Kim and Trump have met three times —a first for any sitting U.S. president — and the two regularly praise each other.

O’Brien declined to discuss whether Washington and Pyongyang have had recent discussions beyond noting that “channels of communication” were open.

"We'll reserve judgment, but the United States will take action as we do in these situations. If Kim Jong Un takes that approach, we'll be extraordinarily disappointed and we'll demonstrate that disappointment." — national security adviser Robert O'Brien

U.S. Sen. Ben Cardin, a Maryland Democrat, said he has seen little progress from Trump’s efforts, and that Kim needed to announce that he plans to give up his nuclear weapons.

“The meetings with the two leaders have produced very little in the denuclearization of the Korean Peninsula," Cardin, a member of the Senate Foreign Relations Committee, said on “Fox News Sunday.”

While Trump in 2018 claimed North Korea was “no longer a nuclear threat” and that he and Kim “fell in love,” a deal between the two countries has remained elusive. Neither side has agreed on any terms of disarmament or the easing of a U.S.-led international sanctions regime.

North Korea suggested a “Christmas gift” would be forthcoming after demanding additional concessions as part of the stalled nuclear talks. Trump played down the threat, saying on Christmas Eve the U.S. would “deal with it” and joking that Kim’s gift could be a “beautiful vase.”