By Phil Stewart

YOKOTA AIR BASE, Japan, Oct 24 - Defense Secretary Leon Panetta ruled out troop cuts in Asia as part of U.S. belt-tightening as he arrived in Japan on Monday, flagging concerns about a "reckless" North Korea and Chinese military assertiveness.

Panetta, on his first trip to Asia since taking over the Pentagon's top job in July, has been assuring allies in the region that the U.S. military will maintain a strong posture in the Pacific despite looming defense spending cuts at home.

"We are not anticipating any cutbacks in this region. If anything we are going to strengthen our presence in the Pacific," Panetta, a former CIA director, told a gathering of U.S. and Japanese forces at Yokota Air Base outside Tokyo.

In an opinion piece he published in a newspaper in Indonesia - his first stop on his Asia tour -- Panetta acknowledged U.S. allies have expressed concern "that America may not follow through on our commitments in this region."

Questions about a potential decline in U.S. military power in the Pacific due to America's fiscal woes come as China's military invests heavily in new technologies, including its aircraft carrier programme, stealth fighter jet and an anti-ship ballistic missile.

But Panetta said the complete U.S. military withdrawal from Iraq this year and the gradual drawdown in Afghanistan would enable the United States to shift more attention to the Asia-Pacific region. They would also allow for more focus on concerns like the threat posed by cyber warfare technologies.

"We have the opportunity now to be able to focus on those challenges," Panetta told the gathering. "Most importantly, we have the opportunity to strengthen our presence in the Pacific. And we will."

Panetta's stop in Japan follows a meeting in Indonesia with defense ministers from the 10-member Association of South East Asian Nations. After visiting Japan, he completes his week-long visit to Asia in South Korea.

ASSERTIVE CHINA

In an opinion piece Panetta wrote for Japan's Yomiuri newspaper, the defense chief noted that China was rapidly modernizing its military "but with a troubling lack of transparency, coupled with increasingly assertive activity in the East and South China Seas."

In comments to U.S. troops in Italy this month, Panetta cited concerns about China as one reason the U.S. military presence in the Pacific was so critical.

"In the Pacific, we're concerned about China. The most important thing we can do is to project our force into the Pacific," Panetta said.

"To have our carriers there, to have our fleet there, to be able to make very clear to China that we are going to protect international rights to be able to move across the oceans freely."

Panetta, in his opinion piece in Japan, also renewed his concerns over North Korea's "reckless and provocative behavior."

His comments coincided with a meeting on Monday and Tuesday in Geneva between representatives from Washington and Pyongyang, although analysts are skeptical wider talks on ending the North's nuclear programs will resume any time soon.

North Korea stands accused of killing some 50 South Koreans in two attacks near their contested maritime border in 2010. It also unveiled a uranium enrichment facility, giving it a second route to making an atomic bomb.

"Together, the U.S. and Japan will work to bring North Korea back to the six party talks, and encourage China to play a responsible role in the international community," Panetta wrote.

In Tokyo, Panetta was expected to press Japanese leaders to move ahead with long-delayed plans to relocate the Marines' Futenma air base to a less populated area of Okinawa island -- reluctant host to around half of the roughly 50,000 U.S. forces stationed in Japan.

The Futenma relocation is part of a broader realignment of U.S. forces that would shift some 8,000 troops to the Pacific island of Guam.

"It's very important that Japan proceed with obviously moving forward with Futenma, getting the appropriate permits that are required," he told reporters in Indonesia on Sunday.

Japan's government wants to submit to Okinawa by year-end an environmental impact assessment needed before the governor of Okinawa can sign off on the base transfer. But there is no guarantee that the governor will agree to the relocation plan even once that happens.

(Editing by Ron Popeski)