White House national security adviser Robert O'Brien tells Axios that the Trump administration has "reached out to the North Koreans" to ask them to resume diplomacy that has been all but dead since October.

Driving the news: O'Brien sat down with Axios at the Eisenhower Executive Office Building on Friday to talk about a range of national security challenges at the start of a new year.

On the tense and dangerous situation with North Korea, O'Brien told us: "We've reached out to the North Koreans and let them know that we would like to continue the negotiations in Stockholm that were last undertaken in early October.

with North Korea, O'Brien told us: "We've reached out to the North Koreans and let them know that we would like to continue the negotiations in Stockholm that were last undertaken in early October. "We've been letting them know, through various channels, that we would like to get those [negotiations] back on track and to implement Chairman Kim's commitment" to denuclearization of the Korean Peninsula.

O'Brien also indicated he was cautiously optimistic about the fact that Kim Jong-un hasn't yet delivered his promised "Christmas gift" — which many analysts expected would be a nuclear weapons test.

Between the lines: It appears President Trump is again trying to lean on what he has described as his warm personal relationship with the brutal North Korean dictator.

Trump recently sent a birthday message to Kim. But the North Koreans have already said Trump's courtship will not change their policy.

So far, Trump's diplomacy has yielded little besides giving Kim more time to expand his nuclear arsenal, according to analysts tracking North Korea's supply of nuclear warheads.

O'Brien's predecessor as national security adviser, John Bolton, recently told Axios the Trump administration is bluffing about stopping North Korea's nuclear ambitions — and should prepare to admit publicly that its policy failed badly.

O'Brien indicated he was hopeful about the implications of Kim's decision — so far — to refrain from firing off a nuclear test in the Christmas and new year period.

Kim "promised to send a Christmas present," O'Brien said. "The president suggested he send him a vase. We didn't get a vase or any other sort of Christmas gift. That appears to be positive."

"All we know is we were told we were going to get a Christmas gift and the Christmas gift didn't come. And so I think that was an encouraging sign. But, again, that doesn't mean we won't see some sort of test in the future," O'Brien added.

The big picture: In four months in office, O'Brien has lived through fresh threats from North Korea, the killing of Iranian Gen. Qasem Soleimani and the rolling crisis with Iran, the killing of ISIS leader Abu Bakr al-Baghdadi, and Trump's controversial withdrawal of U.S. troops from the Turkish-Syrian border, abandoning America's Kurdish allies in the process.

Over that short and intense period, O'Brien has built on what was already a close personal relationship he'd developed with Trump when he led hostage negotiations at the State Department, per officials familiar with their interactions.

O'Brien singled out China as "the No. 1 geopolitical challenge to the United States." And he said the two "biggest threats" to America were from "peer competitors" China and Russia, in keeping with the Trump administration's national defense strategy.

"The United States has never faced a peer competitor like China," O'Brien said. "Because of its population, because of its political system, because of its economy ... and because of its global ambitions and clear goal to dominate world trade and the world economy, China must be taken very seriously."

"We have to be strong enough so that no one would dare challenge us militarily, including the PRC [People's Republic of China]."

More from the interview: