Gardner said he believes any decision on North Korea should be made jointly through the South Korea-US alliance.

"I don't think the United States-Korea relationship has ever been stronger," he said, noting that the US highly values the work the two countries do together on security and economic matters.

The allies' coordination has become especially important as both South Korean President Moon Jae-in and US President Donald Trump have met with North Korean leader Kim Jong-un with a view to dismantling the regime's nuclear weapons program.

"Together through the alliance, developing the approach of maximum pressure has been the appropriate and continues to be the appropriate response," Gardner said, referring to the Trump administration's sanctions campaign against the North. "And the alliance has been very firm in that direction and I'm grateful for that."

The senator said he plans to ask US Secretary of State Mike Pompeo during a hearing next week about any progress that has been made toward denuclearization in the weeks following last month's historic summit between Trump and Kim.

The summit produced a commitment by Kim to work towards the "complete denuclearization" of the Korean Peninsula in exchange for security guarantees from the United States. Pompeo traveled to Pyongyang earlier this month to flesh out that deal, but progress has reportedly been slow.

The South Korean delegation also includes Rep. Kim Sung-tae of the main opposition Liberty Korea Party, Rep. Kim Kwan-young of the Bareunmirae Party, Rep. Chang Byoung-wan of the Party for Democracy and Peace and Rep. Roh Hoe-chan of the Justice Party.

They later met separately with House Foreign Affairs Committee Chairman Ed Royce and House Minority Whip Steny Hoyer.

On Thursday, they plan to meet with Secretary of Commerce Wilbur Ross, House Majority Leader Kevin McCarthy, Deputy Secretary of State John Sullivan, Under Secretary of Defense John Rood and Edward Markey, ranking member of the Senate Foreign Relations Subcommittee on East Asia and the Pacific.

They are scheduled to meet with Deputy US Trade Representative Jeffrey Gerrish on Friday and to attend a lunch meeting hosted by the US Chamber of Commerce, as well as a meeting with Korean Peninsula security experts of the US. (Yonhap)