U.S. Ambassador to the United Nations Linda Thomas-Greenfield makes remarks at Camp Bonifas, the base for the U.N. Command Security Battalion just south of the Demilitarized Zone separating the two Koreas, April 16. Yonhap

The United States is looking at "out of the box" options with South Korea and other countries to continue the sanctions monitoring on North Korea, its top envoy to the United Nations said Tuesday, after Russia's veto on a U.N. mandate meant to renew the sanctions monitoring panel.

Ambassador Linda Thomas-Greenfield made the remark during a visit to the Demilitarized Zone (DMZ) separating the two Koreas, suggesting that "all possibilities" are on the table, even outside of the U.N. regime.

"We're working closely with Korea, with Japan, with other like-minded council members to look at some creative ways, some out of the box thinking, on how we might move forward on continuing the important work that the panel of experts was able to do," she said, when asked by reporters of the prospects of the panel.

"Whether that's the (U.N.) General Assembly, or entities outside of the U.N., I think we have to consider all possibilities."

The U.S. envoy was on a four-day trip to South Korea this week for talks with Seoul officials, as part of her two-nation Asia swing that will also take her to Japan.

Late last month, the U.N. Security Council (UNSC) failed to extend the mandate of the Panel of Experts on monitoring the enforcement of sanctions against the North due to Russia's veto. China abstained in the vote.

Russia is one of the five permanent members of the UNSC that hold veto power, along with Britain, the U.S., China and France.

Moscow's rejection drew criticism that it could undermine international efforts to monitor violations of anti-Pyongyang sanctions aimed at blocking its illicit weapons development. The panel is now set to expire on April 30.

The veto also came amid concerns over the growing military ties between Russia and the North. The U.S. and countries aligned with Washington have accused Pyongyang of providing weapons to Moscow for use in its war in Ukraine in return for Russia's assistance in advanced military technologies.

"It's clear that the reason they're doing that is because of the alliances that they have established with the DPRK," the envoy said, referring to the North by its official name, the Democratic People's Republic of Korea.

Thomas-Greenfield urged Moscow and Beijing to "reverse course" from turning a blind eye on North Korea emboldening its pursuit of illicit weapons programs and evading sanctions.

"Both countries continue to use their positions in the Security Council to shield the DPRK from accountability and most recently to prevent all U.N member states from learning about the DPRK's unlawful weapons program and sanctions evasion efforts," she said.

"We urge Russia and China to reverse course," she said. "Hiding the truth does not change it. Rewarding bad behavior only encourages it."

Thomas-Greenfield reiterated the U.S. call on Pyongyang to return to diplomacy for "constructive" dialogue rather than continuing with its escalatory rhetoric, repeating that Washington harbors no hostile intent toward the North.

"We remain open to dialogue." she said. "All the DPRK has to do is say yes, and show up to the table in good faith."

Her visit marked the first such trip by a top U.S. envoy to the U.N. in more than seven years.

Since Russia's veto on the panel extension, Seoul and Washington have been in talks to explore alternatives, a topic that topped the agenda during Thomas-Greenfield's talks with Foreign Minister Cho Tae-yul on Monday, Cho's office said earlier.

U.S. Ambassador to the United Nations Linda Thomas-Greenfield meets a group of North Korean defectors in Yongsan District, Seoul, April 16. Yonhap

After the visit to the DMZ, Thomas-Greenfield met with a group of North Korean defectors to talk about the North's human rights violations. She was then expected to have a roundtable session with students at Ewha Womans University.

On Monday, she met with Cho and Defense Minister Shin Won-sik.

Thomas-Greenfield will wrap up her trip to Seoul and fly to Japan on Wednesday. (Joint Press Corps-Yonhap)