Washington (CNN) Even as tensions between the US and North Korea have intensified, along with President Donald Trump's rhetoric, US diplomats have quietly and steadily been in contact with their North Korean counterparts.

The meetings, led by the US Special Representative for North Korea Policy Joseph Yun and reported on by CNN, have focused on Americans detained in North Korea, but have also touched on the relationship between the two countries.

The US has been in contact with North Korea since at least February at an official level and there have been lower-level talks between Americans -- though none who represent the US government -- and North Korean officials.

Working through a longstanding mechanism known as the "New York channel" that was created to send messages back and forth through the UN, Yun has been in regular contact with his primary counterpart, North Korea's ambassador to the UN, Pak Song II. The meetings have taken place internationally, including in New York and in Oslo, Norway.

Officials have told CNN that they hoped this diplomatic channel would lead to more fulsome dialogue. Secretary of State Rex Tillerson has emphasized publicly that the US' goal is to start talks with North Korea -- with the important caveat that Pyongyang first relinquish its pursuit of nuclear weapons.

"We felt the appropriate thing to do first was to seek peaceful pressure on the regime in North Korea to have them develop a willingness to sit and talk with us and others," Tillerson told reporters on August 1. "But with an understanding that a condition of those talks is there is no future where North Korea holds nuclear weapons or the ability to deliver those nuclear weapons to anyone in the region, much less to the homeland."

Tillerson has also hinted at other efforts, telling reporters on August 9 that "we have a very active, ongoing diplomatic effort, most of which is behind the scenes because that's where diplomacy is most effective."

On Thursday, Defense Secretary James Mattis praised Tillerson's diplomatic efforts to contain North Korea, saying they were working and that they remain the favored means for solving the crisis. War, Mattis said at a press briefing in Mountain View, California, would be "catastrophic."

'Issues of concern'

Patrick Murphy, Deputy Assistant Secretary for Southeast Asia, was asked about contacts between the US and North Korea on Friday. He wouldn't address specifics, but said, "The North Koreans know how to reach us."

He added a refrain heard often from the administration -- that the US will not negotiate its way to the negotiating table, and that now is not the right time for dialogue because North Korea has not shown a willingness to address issues of concern.

And at a lower level, there have also been various "track two" talks about improving relations between the US and North Korea. These types of talks are often used to manage difficult diplomatic situations and while they don't involve current representatives of the US government or the ruling administration, they establish lines of communication through think tank experts and former officials.

Recent track two talks have brought Americans together with the head of the North America department of the North Korean foreign ministry, an official named Choi Sun Hee, sources tell CNN.

In recent days, hope of more official negotiations taking place seem to have all but evaporated.

Trump tweeted Friday that US military solutions on North Korea are "locked and loaded." Earlier in the week, he had threatened to unleash "fire and fury" on North Korea. Pyongyang's military leaders have countered with highly detailed threats to fire missiles toward Guam. And China and Russia are scrambling to put together a plan that would require both sides to take steps to defuse tensions.

It's a sharp acceleration away from the discreet attempts, led by Yun, to forge some common ground.

After a deep freeze between North Korea and the Obama administration, contacts between Pyongyang and the US picked up again after Trump took office.

Contacts began in February, when Tillerson briefed Trump on the situation with Americans hostages detained in North Korea. Trump directed the Secretary to take all appropriate measures to secure the release of the Americans.

Tillerson began that effort, setting up talks that were supposed to take place in February, but they were canceled after Kim had his half brother assassinated at an airport Malaysia.

Trump later publicly signalled his interest in talks, saying in May that he would be "honored" to meet with North Korean leader Kim Jong Un "under the right circumstances" to defuse tensions over North Korea's nuclear program.

"If it would be appropriate for me to meet with him, I would absolutely, I would be honored to do it," Trump said at the time.

That same month, Yun met high-level representatives from the North Korean Ministry of Foreign Affairs in Oslo, including Choi Sun Hee, the official who leads the North Korean Foreign Ministry's North America department.

At that meeting, the North Koreans agreed to allow the Swedish Embassy in Pyongyang, which represents US interests in the country, to pay a consular visit to all four Americans being held in the country.

It was one of several meetings that Yun held with Pak and other senior level North Korean officials from the foreign ministry since Trump took office. In May, Yun also met with Pak in New York at the North Korean mission to the UN.

There, Yun learned about the health condition of Otto Warmbier , the young American who had gone to North Korea on a short trip, only to be arrested for allegedly stealing a poster. Yun demanded to fly to Pyongyang to see him.

Warmbier was released by North Korea in June but died shortly after he returned to the US in a coma after sustaining severe brain injuries while he was being held by Pyongyang. Three other Americans are still being held in North Korea.

Asked Wednesday about any contacts between North Korea and US officials since that meeting, State Department spokeswoman Heather Nauert said, "not that I'm aware of."