The top US diplomat expressed his gratitude to outgoing Deputy Secretary of State Wendy Sherman on Friday, also praising the first female deputy secretary of state for having deepened US relations with key allies including South Korea and Japan.

Secretary of State Antony Blinken also noted Sherman had broken barriers at the department as the first female deputy secretary.

"Wendy has helped lead our engagement in the Indo-Pacific, the region where the history of the 21st century will be written," Blinken said in a released statement.

"She has deepened our bonds with our friends around the world, especially with the Republic of Korea, Japan, and the European Union," he added, referring to South Korea by its official name.

Sherman's departure comes about two weeks after South Korean President Yoon Suk Yeol made a state visit to the US.

During his six-day trip here, Yoon and US President Joe Biden reached a historic agreement, dubbed the Washington Declaration, to increase the visibility of US strategic assets in and around the Korean Peninsula, which they said will effectively upgrade the countries' 70-year-old Mutual Defense Treaty.

"Wendy broke barriers as our first female under secretary of state for political affairs and our first female deputy secretary of state," Blinken said of Sherman.

"Her remarkable career -- which spans more than three decades, three presidents, and five secretaries of state -- addressed some of the toughest foreign policy challenges of our time. Our nation is safer and more secure, and our partnerships more robust, due to her leadership," he added.