The U.S. ambassador to the United Nations has headed to the demilitarized zone dividing the two Koreas, ramping up pressure on North Korea after sanctions enforcement was dealt a heavy blow by a recent Russian veto.

Ambassador Linda Thomas-Greenfield went to the buffer zone border Tuesday in the highest-profile visit by a Biden administration official since Vice President Kamala Harris went there in 2022.

The 4-kilometer wide buffer zone between the two Koreas, where hundreds of thousands of troops are stationed on opposing sides of razor-wire fencing, is dubbed the Cold War¡¯s last frontier, and a symbol of simmering tensions that have lasted since the end of the 1950-53 Korean War. Thomas-Greenfield is the first U.S. ambassador to the United Nations to visit South Korea since 2016.

Before the visit, Thomas-Greenfield discussed with officials in Seoul ways to extend the mandate of a panel of experts that has monitored North Korea¡¯s nuclear-weapons development for 15 years. Russia vetoed a U.S.-led Security Council resolution last month to extend the panel.

Reports by the body of experts inform decisions on international sanctions established by the Security Council in a series of resolutions aimed at barring North Korea from developing its nuclear arms program. The Russian veto comes as Moscow and Pyongyang draw closer following Russian President Vladimir Putin¡¯s full-scale invasion of Ukraine.

Despite Moscow¡¯s close ties with Pyongyang, the veto marks a shift in policy at the Security Council, being the first time Russia has flatly opposed the panel¡¯s mandate in the 15 years since it was established.

The U.N. Security Council has seen growing divisions over the war in Ukraine, and what the U.S. says is Moscow¡¯s push to secure weapons and ammunition from Kim Jong Un¡¯s regime for the Kremlin¡¯s assault on its neighbor.

Moscow and Pyongyang have denied the charges despite a multitude of satellite photos released by research groups, and the U.S. showing the flow of weapons from North Korea to Russia and then to munitions dumps near the border with Ukraine.

In return for the arms, Russia is providing North Korea with food, raw materials and parts for weapons production that help Kim boost food security and further develop his weapons systems, South Korean officials have said.

Thomas-Greenfield arrived in Seoul on Sunday for a four-day visit that will also include a meeting with North Korean defectors before she heads to Japan.

In Japan, the U.S. ambassador will meet officials to discuss shared priorities at the U.N. and hear from family members of Japanese citizens abducted by North Korea, her office said.