US Defence Secretary Chuck Hagel on Saturday vowed America would stand by its security commitments to Tokyo, after Russia's intervention in Ukraine raised concerns in a region plagued by its own territorial disputes.

Hagel, on a two-day visit to Japan as part of a tour of Asia, acknowledged Russia's annexation of Crimea had sparked worries among allies in the Pacific and elsewhere.

Japan is locked in a bitter dispute with China over islands in the East China Sea, and some analysts have warned that Russia's move in Ukraine could embolden Beijing or other powers to take unilateral action to settle territorial claims.

"It's a pretty predictable ... reaction, not just of nations in this area, in this region, but all over the world. It has to concern nations," Hagel told reporters before landing at Yokota Air Base in Japan.

Although his trip was planned long before Russia's incursion, Hagel said "another reason I'm here is to reassure our allies of our commitments to their security".

He said US defence ties with Tokyo had strengthened in recent years with the deployment of advanced surveillance aircraft to the country and plans to station a second early-warning radar there later this year.

"I don't think there's any indication or any evidence that we're doing anything but strengthening our commitment to the security of Japan," he said.

Hagel later reinforced the message speaking to a group of US and Japanese troops in a hangar at Yokota Air Base, saying he had come to reaffirm "our continued commitment to our partnership, our friendships and our treaty obligations."

"We are serious about that," he said, standing in front of giant US and Japanese flags.

Soaring tensions in the East China Sea have prompted the United States to explicitly state its mutual defence treaty with Tokyo applies to the islets, which are currently administered by Japan.

Washington has warned China against trying to resolve the disagreement through force.

The crisis over Ukraine came up at a gathering of ASEAN defence ministers hosted by Hagel earlier this week in Hawaii, a defence official said.

But the topic was raised only "tangentially" and "there wasn't a lot of hand wringing over Ukraine and what will happen in their part of the world," the official, who spoke on condition of anonymity.

- Territorial disputes a 'concern' -

At the end of the ASEAN session, Hagel on Thursday voiced "increasing concern" over separate territorial disputes between several nations in the South China Sea, which Beijing claims almost in its entirety.

The Philippines and some other Southeast Asian states have accused China of aggressively asserting its claims and Hagel -- without openly referring to Beijing -- said there was no place for "bullying."

"You always have concerns when these issues continue to play out and drag out. And you always have concerns when there's any indication of coercion, intimidation, or bullying," he told reporters travelling on his plane.

He said the United States did not take a position on the specific territorial arguments and favoured resolving them peacefully.

But he added: "That said, we will honour all of our treaty commitments to our treaty partners."

Hagel also said one of the top themes on his agenda was promoting stronger ties between Japan and South Korea, which have been severely strained in recent months as controversies have flared over the legacy of Japan's 1910-45 colonial rule.

Hagel is due to meet Prime Minister Shinzo Abe on Saturday before holding talks on Sunday with his counterpart, Itsunori Onodera, and Foreign Minister Fumio Kishida.

In an article published by Japan's Nikkei daily, Hagel said he supported the Japanese prime minister's plan to review Japan's self-imposed ban on defending allies under armed attack.

Japan's conservative premier has spoken repeatedly of his desire to revise the US-imposed pacifist constitution and is pushing to broaden the role of the military to permit "collective self-defence", allowing Japanese troops to come to the aid of allies.

The Pentagon chief's tour, which will take him next to China and Mongolia, comes ahead of a trip to Asia by President Barack Obama later this month.