WASHINGTON — The top U.S. military commander in the Pacific expressed concern Tuesday at China's huge boost in military spending and said it will take considerable effort to reverse years of mistrust between Washington and Beijing.

Adm. Timothy Keating told lawmakers at a Senate Armed Services Committee hearing that China's rising defense spending threatens U.S.-ally Taiwan and U.S. capabilities in Asia.

U.S.-Chinese military relations, he said, have fallen short of expectations, with China showing little willingness to conduct port visits, sea exercises or midlevel officer exchanges. He noted what he called China's "perplexing cancellation" of port calls for U.S. ships in Hong Kong last year.

Despite some high-level U.S. visits to China, including by the U.S. secretary of defense, "there are miles to go" before the relationship improves, Keating said.

"The breakdown of decades-old mistrust and custom is going to take a lot more effort," Keating said.

Beijing has announced a nearly 18 percent boost in defense spending. The increase this year to nearly $59 billion is the 18th double-digit percentage increase in 19 years.

Keating urged China to better explain its military spending. "Being able to see what they have doesn't tell us what they intend to do," Keating said.

China insists the buildup is defensive.

Lawmakers also expressed alarm.

"These people have got to remove themselves from the Dark Ages if they want to be respected, I think, by other military powers," said Republican Sen. John Warner of Virginia, referring to China.

Warner said he was also worried that Taiwan thinks the U.S. obligation to maintain sufficient force in the Pacific as a deterrent against China will serve as "a 911 dial to the United States to come rescue us."

China claims the democratic, self-governing island as its own and has threatened to attack should Taiwan formalize its de-facto independence.

Keating, however, told lawmakers it is "very unlikely that anything will happen" between China and Taiwan.

Taiwan's candidates in March 22 presidential elections have a more moderate stance on China, he said, making the U.S. cautiously optimistic "that a little bit of steam will leave the kettle" after the vote. Current Taiwan President Chen Shui-bian has pushed a vision of an independent Taiwan.

Despite consternation over China's defense spending, there has been a recent warming of U.S.-Chinese military ties. China recently agreed to allow access to sensitive records on American servicemen missing since the 1950-53 Korean War. The two countries also agreed to set up a military hot line for communicating in emergencies.

On North Korea, Keating said the U.S. military is "very, very, very cautiously optimistic" about international efforts to persuade Pyongyang to abandon its nuclear programs. Six-nation negotiations are currently stalled, with the United States saying North Korea has not handed over a promised declaration of the details of its nuclear weapons program.

Army Gen. B.B. Bell, the top U.S. commander in South Korea, told lawmakers that his assessment is that North Korea "will continue to resist fundamental change, focusing its international engagement, strategic dialogue and military readiness to ensure its long-term survival."