The United States and North Korea have been communicating through diplomatic back channels amid rising tensions between the two countries over Pyongyang's missile tests and nuclear program, according to The Associated Press.

It was known that U.S. envoy to North Korea Joseph Yun and North Korean diplomat Pak Song Il had negotiated the release of imprisoned Ohio college student Otto Warmbier. But what was not known is that the diplomats have talked since then, the AP reports.

So far, the talks have done little to bring the two countries together, according to AP, and Yun is the only American envoy in contact with a North Korean counterpart.

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Tensions between the U.S. and North Korea have steadily climbed since Warmbier's death and Pyongyang's launch of two intercontinental ballistic missiles (ICBM) in July.

U.S. intelligence agencies now assess that Pyongyang has successfully miniaturized a nuclear warhead, putting the country one step closer to becoming a nuclear power.

Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE has stressed that the Trump administration is hoping for a diplomatic solution to North Korea, with China and Russia convincing North Korea to "reconsider the current pathway they’re on and think about engaging in a dialogue about a different future."

But amid that call for diplomacy, President Trump and other members of the administration have ratcheted up their rhetoric.

“North Korea best not make any more threats to the United States,” Trump told reporters on Tuesday.

“He has been very threatening beyond a normal state, and as I said they will be met with fire, fury and frankly power, the likes of which this world has never seen before," he continued.

North Korea responded with a specific threat, saying it would attack the waters around the U.S. territory of Guam.

Trump doubled down Thursday on his warning to North Korea.

“Frankly, the people who were questioning that statement, was it too tough? Maybe it wasn’t tough enough,” Trump told reporters at his New Jersey golf club.

“They’ve been doing this to our country for a long time, for many years, and it’s about time that somebody stuck up for the people of this country and for the people of other countries.”