United States Defense Secretary Jim Mattis has proposed maintaining around-the-clock communication with South Korean Defense Minister Han Min-koo in a bid to tighten the countries' alliance further against North Korea, a military source said Sunday.

"Secretary Mattis suggested to Minister Han (they) communicate closely around the clock and all year round during their defense ministerial talks on Friday," the source said, referring to the defense chiefs' recent meeting in Seoul.

"Han responded that he agreed to the proposal eagerly," the source noted.

Mattis paid a two-day visit to Seoul earlier this week, his first overseas trip since taking office, as he tries to reinforce the alliance with South Korea following President's Donald Trump's berating remarks on Seoul's defense cost sharing.

The latest proposal reflects Mattis' efforts to ensure a seamless alliance with Seoul at a time when North Korea has threatened to carry out an intercontinental ballistic missile test.