A top US diplomat working with North Korea reportedly says diplomacy is on its "last legs" as President Donald Trump refuses to accept a nuclear-armed Pyongyang.

Trump has been clear that the US will take steps to prevent North Korea developing a nuclear missile capable of hitting the US, and Pyongyang has been clear it will not stop until it gets one.

The atmosphere between the two nuclear countries is dense as ever as Secretary of State Rex Tillerson says "diplomatic efforts will continue until the first bomb drops."

Multiple US government and congressional sources have said US diplomatic efforts with North Korea stand on their "last legs" as Pyongyang threatens more nuclear tests and Washington ponders all-out war.

The sources told NBC News on Wednesday that the efforts of Joseph Yun — a top US diplomat working with North Korea — to find a diplomatic solution to the missile crisis have all but completely broken down.

A US official told NBC that the sides have reached an impasse, as North Korea apparently pursues completing its nuclear missile program above all else, and the US repeatedly states that it won't tolerate a nuclear-armed North Korea.

"It is not so much that North Korea is shutting down, it’s that the message from the US government is, 'surrender without a fight or surrender with a fight,'" the official said. Another added that diplomatic efforts are on their "last legs."

Yun went as far as echoing US Senator Bob Corker, the Republican from Tennessee who came out against President Donald Trump's leadership in recent weeks, by telling congressional aides and officials that the White House has "handicapped" diplomacy.

While Trump has publicly criticized Secretary of State Rex Tillerson's attempts to engage with North Korea, a report from CNN indicated that North Korea wouldn't negotiate with the US until it had completed more tests of its nuclear delivery systems that can reach the US.

In any case, the stalemate has taken on ominous overtones, with Trump repeatedly hinting at military action and Tillerson saying that "diplomatic efforts will continue until the first bomb drops."