Donald Trump's defence secretary James Mattis, left, is to visit Asia
The trip is the first for retired Marine General James Mattis since becoming President Trump's Pentagon chief and is also the first foreign trip by any of Mr Trump's cabinet secretaries.
Officials say the fact that Secretary Mattis is first heading to Asia – as opposed to perhaps visiting troops in Iraq or Afghanistan – is meant to reaffirm ties with two Asian allies hosting nearly 80,000 American troops and the importance of the region overall.
That US reaffirmation could be critical after President Trump appeared to question the cost of such U.S. alliances during the election campaign. He also jolted the region by pulling Washington out of an Asia-Pacific trade deal that Japan had championed.
"It's a reassurance message," said one Trump administration official, speaking on condition of anonymity.
"This is for all of the people who were concerned during the campaign that then-candidate, now-president, Trump was skeptical of our alliances and was somehow going to retreat from our traditional leadership role in the region."
President Trump himself has spoken with the leaders of both Japan and South Korea in recent days and will host Japanese Prime Minister Shinzo Abe in Washington on February 10.
It's a reassurance message
Trump administration official
Secretary Mattis leaves the United States on February 1, heading first to Seoul before continuing to Tokyo on Februsry 3.
President Trump singled out both South Korea and Japan on the campaign trail, suggesting they were benefiting from the US security umbrella without sharing enough of the costs.
In one 2016 television interview, President Trump said of the 28,500 US troops deployed to South Korea: "We get practically nothing compared to the cost of this. Why are we doing this?"
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US President Donald J. Trump attends a meeting on cyber security, in the Roosevelt Room of the White House, in Washington, DC, USA, 31 January 2017
It will by Marine General James Mattis' first trip since becoming Trump's Pentagon chief
Secretary Mattis, in his confirmation hearing, appeared to play down those remarks, noting that there was a long history of U.S. presidents and even defence secretaries calling on allies to pay their fair share of defence costs.
But his visit to the region comes amid concerns North Korea may be readying to test a new ballistic missile, in what could be an early challenge for Trump's administration.
Speaking with South Korean Defence Minister Han Min-koo ahead of his trip, Mattis reaffirmed a U.S. commitment to defend the country and "provide extended deterrence using the full range of US capabilities."
'Mad Dog' Mattis is expected to underscore the US commitment to allies South Korea and Japan
Analysts expect Secretary Mattis to seek an update on South Korea's early moves to host a Terminal High Altitude Area Defense (THAAD) system, which, once in place sometime in 2017, would defend against North Korea's nuclear and ballistic capabilities.
Still, a South Korean military official played down expectations of any big announcements during the trip, saying Secretary Mattis's first visit would likely be "an ice-breaking session" for both countries.
In Tokyo, Secretary Mattis is to meet Defence Minister Tomomi Inada, who has repeatedly said Japan is bearing its fair share of the costs for U.S. troops stationed there and has stressed that the alliance is good for both nations.
Donald Trump will host Prime Minister Shinzo Abe in Washington on February
Japan's defence spending remains around 1 percent of GDP, far behind China, which is locked in a dispute with Japan over a group of East China Sea islets 140 miles northeast of Taiwan known as the Senkakus in Tokyo and the Diaoyus in Beijing.
The trip also comes amid growing concern about China's military moves in the South China Sea. Tension with Beijing escalated last week when Trump's White House vowed to defend "international territories" there.
China responded by saying it had "irrefutable" sovereignty over disputed islands in the strategic waterway.
"What US military people say is that considering the pace of China's military build-up such as anti-ship missiles and fighters, there are worries about Japan's capabilities," said a senior Japanese defence ministry official, speaking on condition of anonymity.