WASHINGTON -- The United States and North Korea should break their impasse in nuclear negotiations by establishing liaison offices in each other's capital and broadening the scope for diplomacy, a former US envoy on Pyongyang said Wednesday.

The pitch by Joseph Yun, who retired as US special representative for North Korea policy earlier this year, comes as Washington and Pyongyang have been deadlocked over the implementation of the Singapore summit agreement reached by US.

President Donald Trump and North Korean leader Kim Jong-un in June.

The agreement commits Kim to work toward the "complete denuclearization" of the Korean Peninsula in exchange for security guarantees from the US.

In an opinion piece in The Washington Post, Yun said the current impasse could lead to an "all-too-familiar crisis cycle."

"To avoid this outcome, the (Trump) administration must broaden the diplomatic process," he wrote. "The most effective way to do this would be to establish reciprocal diplomatic liaison offices in Pyongyang and Washington. This would send a strong signal that the United States is prepared to meet the first commitment from the Singapore summit, while seriously testing North Korea's insistence that it is looking for 'new relations' with Washington."

The first commitment made in the joint statement was to "establish new US-DPRK relations," where DPRK is an acronym for North Korea's formal name, the Democratic People's Republic of Korea.

Yun cited the success of past US liaison offices in former adversaries such as China and Vietnam. The small diplomatic presence in those two countries helped resolve bilateral issues and eventually paved the way for full-fledged embassies in Beijing and Hanoi, he said.

North Korea also made an attempt at establishing a liaison office in Washington in the 1990s but scrapped the plan under the "paranoid and reclusive" then-leader Kim Jong-il, Yun noted, adding that "the outcome looks more likely under his worldlier son," current leader Kim Jong-un.

"It is now time for the Trump administration to revisit that (summit) agreement in order to broaden the diplomatic process with North Korea," the former envoy said. "Clinging to its own interpretation of the Trump-Kim agreement and its one-track denuclearization demand clearly is not working."

Yun traveled to Pyongyang last year to bring home American college student Otto Warmbier, who was detained in North Korea for removing a political poster from a hotel.

Warmbier was found in a coma and died shortly after his return to the US. (Yonhap)