BEIJING - The United States has been surprised by the pace of China's military development, Secretary of Defense Robert Gates said Saturday, acknowledging that U.S. intelligence has underestimated elements of the country's military modernization.

Gates spoke on board his plane as he headed to China on a mission to further patch together ties with China's military. Gates last visited China in 2007, and subsequently China suspended military contacts - twice - following U.S. decisions to sell arms to Taiwan.

Gates' trip comes in the run-up to the second summit between President Obama and China's President Hu Jintao, due to start Jan. 18 when Hu arrives in Washington. Relations between Beijing and Washington went through a rough patch last year following the arms sales and other signs of growing strategic competition, but lately have warmed.

Echoing other U.S. officials, Gates praised China for its apparent role in pressing North Korea to halt - at least for now - its attacks on South Korea, which Gates will also visit. The Korean Peninsula reached its tensest point in years in November after North Korea launched an artillery attack on a South Korean island, killing two civilians and two soldiers. In March, a North Korean submarine is believed to have sunk a South Korean warship, killing 46 sailors.

"Certainly from our standpoint we recognize that China played a constructive role in lessening tensions on the peninsula," Gates said.

Gates said the goal of his trip was to try to get China to engage in a serious strategic dialogue with the United States in order to avoid miscalculations that could lead to conflict. One area that concerns the United States is China's military modernization program. Most, if not all, of its new weapons systems seemed designed with one enemy in mind - the United States.

Last week, photographs appeared on Chinese Internet sites of what is apparently China's first stealth fighter during a runway test in western China. In December, Adm. Robert Willard, commander of U.S. forces in the Pacific, told a Japanese reporter that China had made significant advances in the development of an anti-ship ballistic missile - designed to destroy an aircraft carrier.

"We've been watching these developments all along," Gates said, briefing reporters. "I've been concerned about the development of the anti-ship ballistic missile ever since I took this job" in 2007.

Gates intimated, however, that the U.S. government was surprised about the stealth fighter, called a J-20.

"We knew they were working on the stealth aircraft," he said. "What we've seen is that they maybe are somewhat further ahead in the development of that aircraft than our intelligence had earlier predicted."

But in another sign that U.S. intelligence lacks concrete information about China's military changes, Gates said it was unclear how good China's stealth technology is.