WASHINGTON -- South Korea's national security advisor Suh Hoon has been in Washington on a previously unannounced trip and has already met with his US counterpart Robert O'Brien, multiple sources confirmed Wednesday.

Suh arrived in Washington on Tuesday, an informed source said on condition of anonymity without providing further details, including what the purpose of his trip was.

O'Brien later confirmed Suh's visit, saying in a tweet that they met earlier in the day.

"Great to see my friend and colleague ROK National Security Office Director Suh Hoon at the White House today," O'Brien said, using the acronym for South Korea's official name, the Republic of Korea. "Our ironclad alliance is stronger than ever, and continues to grow to meet all regional and global challenges."

Secretary of State Mike Pompeo is scheduled to meet with Suh on Thursday, according to a daily schedule of Pompeo released by his department.

Further details of Suh's trip were not immediately available.

Suh's unannounced visit comes after South Korean President Moon Jae-in called for international efforts to declare a formal end to the 1950-53 Korean War in his speech at the United Nations General Assembly last month.

Moon believes the move would help resume the stalled negotiations over the denuclearization of North Korea.

US Deputy Secretary of State Stephen Biegun earlier called the initiative one of the "creative" ways to move forward.

South and North Korea technically remain at war as the 1950-53 war ended only with an armistice, not a peace treaty.

Pyongyang has been staying away from denuclearization talks since its leader Kim Jong-un's second bilateral summit with US.

President Donald Trump, held in Hanoi in February 2019, ended without a deal.

Suh's US trip also coincides with the annual defense ministerial talks between South Korea and the United States, which were held earlier in the day in Washington.

South Korean Defense Minister Suh Wook arrived here Tuesday but was set to head home shortly after his meeting with US Defense Secretary Mark Esper. (Yonhap)