WASHINGTON, Jan. 29 (Xinhua) — The U.S. and South Korean presidents spoke on the phone Sunday and agreed to take steps to strengthen their countries' joint defense capabilities.

In the telephone conversation with South Korean Acting President Hwang Kyo-ahn, U.S. President Donald Trump reiterated America's "ironclad" commitment to defending South Korea, including through the provision of extended deterrence, the White House said in a statement.

They also discussed Pentagon chief James Mattis' upcoming visit to South Korea, noting that "it reflects the close friendship between our two countries and demonstrates the importance of" the U.S.-South Korea alliance, added the statement.

Mattis is set to travel to Japan and South Korea for his first overseas visit since taking office as the U.S. secretary of defense.

Departing on Wednesday, he will begin his trip in South Korea, where he will meet with his South Korean counterpart, Han Min-koo, and other senior officials. On Friday, Mattis will travel to Tokyo to meet with his Japanese counterpart, Tomomi Inada.