The US ambassador to the United Nations visited the heavily-fortified border between North and South Korea Tuesday, urging Pyongyang to return to talks as global enforcement of UN sanctions stumbles.

Linda Thomas-Greenfield arrived in South Korea on Sunday on a trip aimed at keeping up pressure on the nuclear-armed North after Russia last month used its UN veto to effectively end UN monitoring of violations of the raft of sanctions on Kim Jong Un's regime.

Experts have said the shift was a significant victory for Kim, who has recently ramped up ties with Moscow including, Washington and Seoul have claimed, sending Russia weapons for use in Ukraine.

"The United States harbours no, no hostile intent towards the DPRK," Thomas-Greenfield said at the demilitarized zone (DMZ) between the two Koreas, referring to the North by the acronym of its official name.

"We have held the doors open for meaningful diplomacy and we remain open to dialogue, real, productive dialogue without preconditions," she said.

"All the DPRK (has) to do is say yes and show up to the table," she added.

Washington and Seoul have condemned Moscow for its move at the United Nations, calling it "irresponsible".

Kim met Russian President Vladimir Putin in September last year, with Pyongyang's leader declaring Moscow ties his country's "number one priority".

Seoul has since claimed that the North has sent 7,000 containers of arms to Russia.

"This is clearly a concern of ours," Thomas-Greenfield said, when asked about North Korea-Russia ties.

"It is certainly the reason we're seeing Russia protect the DPRK in the council vetoing the 1718 Panel of Experts resolution, blocking efforts to hold the DPRK accountable for numerous violations of resolutions in the council," she said.

This year, Kim has declared Washington's security ally Seoul his country's "principal enemy", jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

Pyongyang has also been making efforts to strengthen ties with its most important ally, Beijing. Last week, it received a visit from China's third highest-ranking official and called the bilateral relationship an "eternal friendship."

"We urge Russia and China to reverse course and once again to urge Pyongyang to choose diplomacy and come to the negotiating table," Thomas-Greenfield said.

The DMZ has been a regular stop for US presidents visiting the South.

Former president Donald Trump and Kim in 2019 had an impromptu meeting in the DMZ -- after Trump issued an invitation on Twitter the day before -- though the meeting did not yield any significant breakthroughs.

South Korea's former president Moon Jae-in and Kim held two of their three summits in 2018 at Panmunjom, a "truce village" within the DMZ.

Moon's diplomacy efforts however ultimately failed to denuclearise the North.