China and the United States remain starkly different on military issues and have a long way to go toward building a trusting relationship, the top U.S. military officer said Friday after a bumpy visit to Beijing.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said he believed his talks with Chinese military leaders were "productive and generally positive." But he added that efforts to create a working military-to-military relationship are still young and so far fraught with difficulties.

"There is a long way to go," he said in a news conference at the U.S. Embassy in Tokyo, where he is wrapping up his Asian tour. "Differences between us are still stark."

Mullen's visit to China was the first of its kind in four years. It was intended to build on efforts to increase communications and exchanges between the two in hopes of easing growing tensions over China's growing military might and economic clout in the region.

Mullen said he was pleased he was afforded access to his counterpart, Chen Bingde, chief of the General Staff of the People's Liberation Army, and given a look at some of China's technology and bases.

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But he said the visit also underscored the wide rift between the two nations.

In a joint news conference during Mullen's stay in China, Chen chided the United States for pouring too much money into its military in a time of economic recession. He suggested that fears of a Chinese threat are overstated.

Chen, who made a similar trip to the U.S. in May, stressed that China remains well behind the United States in military capability and said its military growth is purely for defense.

Mullen on Friday said he was not convinced.

"It's too early to say where China is going with all of this," he said. "They say it's defensive. We'll see."

Mullen also said that the activities of the Chinese, particularly regarding freedom of navigation in the South China Sea, have served to fuel concerns over its ultimate intentions, which he said remain opaque.

Over the past year, China has seen a flare-up in territorial spats with Japan, the Philippines and Vietnam and seen its relations strained with South Korea -- all of which have turned to Washington for support.

Mullen said Washington is also concerned about developments in Chinese missile technology, its activities in the cyber world, and its military satellite capabilities.

He stressed, however, that the United States is not going to withdraw from the region.

"As I told the Chinese, the United States isn't going away," he said. "We've operated in the South China Sea for many decades, and we will continue to do that."