South Korea's President Moon Jae-in pledged Thursday to upgrade the country's alliance with the United States in a phone call with President Joe Biden, Seoul's presidential office said.

The call comes as Moon, who had offered to be a mediator between the United States and North Korea, faces the challenge of getting the stalled talks between Washington and Pyongyang back on track.

North Korean leader Kim Jong Un and former President Donald Trump agreed to work toward the denuclearization of the Korean Peninsula at their first summit in 2018, but a second summit and subsequent working-level talks fell apart.

"We will always stand together as we work for peace on the Korean Peninsula and tackle global challenges," Moon said in a statement after his 8 a.m. call with Biden.

Moon also welcomed what he described as "America's return" in the midst of mounting global challenges – the COVID-19 pandemic, climate change and economic polarization.

South Korea's unification minister in charge of inter-Korean affairs called Wednesday for the United States to flexibly enforce sanctions aimed at curbing Pyongyang's nuclear programs to revive denuclearization talks.