Mark Lippert, the U.S. ambassador to South Korea, said Monday that the country's alliance with South Korea is "ironclad" in the face of North Korea's latest nuclear test.

"The U.S.-South Korea alliance is unshakable and strong like an ironclad," Lippert said during a meeting with lawmakers at the National Assembly.

The National Assembly's foreign affairs committee invited Lippert to discuss actions by Seoul and Washington on North Korea's fourth nuclear test last week.

Lippert also noted that B-52 bomber over South Korea is another sign that the alliance remains staunch.

The United States' B-52 Stratofortress strategic bomber flew across the skies of South Korea on Sunday in a major show of force.

Pyongyang asserted on Wednesday that it has successfully tested a hydrogen bomb, a claim disputed by outside experts but that still raises concerns about the North's advances in weapons technology.

In response, Rep. Na Kyung-won, chairwoman of the committee, raised the need of deploying more U.S. strategic assets on the Korean Peninsula as well as sharing more information between the two countries.

"The international community especially should urge the United Nations to adopt a sanction against North Korea," Na said.

A new sanctions resolution is being discussed at the U.N. Security Council after its earlier resolutions over the North's three previous nuclear tests apparently failed to rein in the country's nuclear weapons program.

Separately, Russian Ambassador to Seoul Alexander Timonin told lawmakers that North Korea should pay for its action, including a further U.N. sanction.

"(Russia) is deeply concerned about the fourth nuclear test and believes that it is the violation of the U.N. resolution," the ambassador said during a meeting with the lawmakers.

(Yonhap)