U.S. Defense Secretary James Mattis said in Tokyo Friday the U.S. stands "firmly, 100 percent, shoulder to shoulder" with Japan.
Mattis, on his first trip since becoming the Pentagon chief, made the comment during a meeting with Japanese Prime Minister Shinzo Abe.
WATCH: Mattis remarks during Japan stop
Earlier Friday in South Korea, Mattis said any nuclear attack by North Korea on the U.S. or any of its allies would be met with an "effective and overwhelming" response.
His trip to South Korea and Japan is to reassure the two Asian allies of Washington's enduring alliance with them.
U.S. President Donald Trump threatened during his campaign to withdraw American forces from South Korea and Japan if they did not pay more for the military support they received from the U.S. South Korea has 28,500 U.S. troops stationed there, while 47,000 U.S. troops are based in Japan.
Mattis also visited Seoul's National Cemetery Friday where he and his South Korean counterpart Han Minkoo participated in a wreath-laying ceremony to pay tribute to the soldiers who died in the Korean War.
On Thursday, Mattis said the Trump administration is committed to strengthening relations with South Korea in the face of what he called the "provocations" Seoul faces from North Korea.
"Right now we have to address the reality of the threat that your country and my country faces, and we intend to be shoulder-to-shoulder with you as we face this together," he said.
Mattis spoke alongside South Korean Prime Minister Hwang Kyo-ahn at the start of Mattis' first overseas trip as Pentagon chief.
Hwang said he looks forward to further consultations on the U.S.-South Korea alliance and "responding to North Korea's nuclear issue."
Before landing in South Korea, Mattis told reporters traveling with him that one topic of conversation during his visit will be the THAAD missile defense system, which the U.S. and South Korea want to deploy this year over the objections of China.
"Were it not for the provocative behavior of North Korea, we would have no need for THAAD out here," Mattis said. "There is no other nation that needs to be concerned about THAAD other than North Korea if they are engaged in something that is offensive."