James Mattis is due to travel to South Korea on Wednesday and Japan on Friday on his first trip as defense secretary.
"President Trump reiterated our ironclad commitment to defend the ROK, including through the provision of extended deterrence, using the full range of military capabilities," the White House said in a statement, using an acronym for the South's formal name.
"The two leaders agreed to take steps to strengthen joint defense capabilities to defend against the North Korean threat."
Pentagon chief James Mattis is due to travel to South Korea on Wednesday and Japan on Friday on his first trip as defense secretary.
The trip comes amid worries in the two long-standing American allies about the direction of US policy in their region under President Donald Trump.
During his campaign, Trump threatened to withdraw US forces from the two countries if they did not step up their financial support for their defense.
But the White House insisted that the trip "reflects the close friendship between our two countries and demonstrates the importance of the US-ROK alliance."
Seoul and Washington agreed last year to install the Terminal High Altitude Area Defense (THAAD) system in the South after a string of North Korean nuclear and missile tests -- prompting strong objections from China, which fears it will undermine its own ballistic capabilities.
Earlier this month, Hwang warned that North Korea's nuclear and missile capabilities are accelerating at an "unprecedented" pace, as he called for the "swift" deployment of the anti-missile system.
Within South Korea, voices opposing the THAAD installation have grown louder, with some opposition candidates pledging to scrap the agreement if they win a presidential election due this year.
The plan has also angered Beijing, which has imposed a string of measures seen in the South as economic retaliation, including effectively barring K-pop stars from performing on the mainland and not authorizing South Korean airlines to operate charter flights between the countries.