U.S. Secretary of Defense Chuck Hagel speaks to both U.S. and Japanese troops on Saturday, April 5, 2014 at Yokota Air Force Base in Fussa, Japan. (Alex Wong/AP)

On his fourth trip to Asia as secretary of defense, Chuck Hagel is attempting to reassure allies in a region brimming with territorial disputes amid concerns about Russia’s takeover of Crimea.

Spooked by the speed and ease with which Russia annexed the peninsula last month, close U.S. partners in the region are questioning the strength of security cooperation agreements with Washington.

Addressing U.S. and Japanese troops at Yokota Air Base after landing here Saturday afternoon, Hagel said the United States remains committed to its “partnership, our friendship and our treaty obligations” with Tokyo. “We’re serious about that.”

That’s a message Japanese officials are eager to hear stated clearly and publicly after China alarmed its neighbors last year by declaring an “air defense identification zone” over a territory that includes an uninhabited archipelago both nations claim rights to.

On Thursday, during the last session of a three-day summit with Southeast Asian defense ministers in Hawaii, Hagel said the United States had grown “increasingly concerned” about the region’s territorial disputes.

“The rights of all nations must be respected,” he said at the closing of the conference in Honolulu. “It’s important that all nations avoid the use of threats or force or intimidation, coercion.”

Hours after landing in Japan on Saturday afternoon, Hagel met with Prime Minister Shinzo Abe, whose government is spearheading an effort to strengthen and expand Japan’s security forces. Hagel is scheduled to meet the country’s defense minister Sunday. Speaking to reporters on the flight to Japan, the Pentagon chief said he understands the unease that the situation in Crimea has caused among U.S. allies.

“It’s a pretty predictable reaction not just of nations of this area, but all over the world,” he said. “Any time you have a nation — Russia in this case — try to impose its will to redefine international boundaries and violate the territorial integrity and sovereignty of a nation by force, all of the world takes note of that.”

The fact that the United States has ruled out a military response to the crisis in Crimea should not be considered a sign of wavering commitment to agreements such as the U.S.-Japan Security Treaty, which obligates Washington to defend Japan, Hagel said.

There is no “weakness on the part of the United States as to our absolute commitment to the security of Japan,” he told reporters.

During the trip, which will include stops in China and Mongolia, Hagel will try to reassure leaders in the region that the Obama administration remains committed to shifting more resources and attention to the Asia-Pacific region as an era of ground wars comes to an end.

However, senior military leaders have expressed concern about their ability to respond to crises in the region in light of defense budget cuts and long-term financial uncertainty. Adm. Samuel J. Locklear III, the head of the U.S. Pacific Command, recently testified before Congress that the United States would be ill-equipped to execute a successful amphibious warfare campaign if China and Japan went to war over the disputed islands.

In a separate hearing, Adm. Jonathan Greenert, the U.S. chief of naval operations, said he worries about being able to keep up with China’s military growth.

“I’m very concerned” about “our ability to project power in an area against an advanced adversary with those advanced capabilities,” he said. “We’re slipping behind.”