The United States needs to remain closely engaged militarily with its Asian and Pacific allies, especially at a time when the region is watching with concern Russia's behavior toward neighboring Ukraine, the country's top Air Force commander for the Asia-Pacific region said on Monday.

"The importance of the Asia-Pacific to the future of the United States I don't think can be overstated," U.S. Air Force Gen. Herbert Carlisle told a discussion at the Center for Strategic and International Studies in Washington, more than two years after the United States announced its Asia-Pacific strategic rebalance, which includes plans to deploy 60 percent of its warships to the region by 2020 and an increase in American troop rotations there.

Carlisle noted that for Asian-Pacific nations dealing with territorial or internal disputes, Russia's intervention in neighboring Ukraine and its annexation of Crimea are causing increasing concerns.

"What Russia is doing in Ukraine and Crimea has a direct effect on the Asia-Pacific," he said, adding that Russia is becoming "increasingly active" in the Pacific, where the country has a long Eastern border, making its actions "a challenge for us in the Asia- Pacific as well as Europe."

As area air defense commander, Carlisle's command of 45,000 Pacific Air Forces airmen has him overseeing security for more than half the planet's surface and the world's five largest economies, as well as 60 percent of the planet's population. "You can fit every land mass on the planet in the Pacific Ocean and still have room left over for another North American continent and another African continent," the general noted.

Last year's federal budget sequester had the effect of slowing down the number of U.S. military regional engagements, Carlisle said, but that has since picked up, with the total number set to be about 400 this year. But he warned that the challenges to regional security continue to exceed the capacity of the United States.