KADENA AIR BASE, Japan — While the U.S. has been tied up in Iraq, China is modernizing its military and its air defenses are now nearly impenetrable to all but the newest of American fighters, the senior U.S. military official in Japan said.

Lt. Gen. Bruce Wright, commander of the roughly 50,000 U.S. forces in Japan, Washington's biggest ally in Asia, said this week that the Iraq war is reducing the availability of U.S. troops and equipment to meet other contingencies.

It's also eating funds that could go toward replacing or upgrading planes, he said.

China, meanwhile, is rapidly filling the skies with newer, Russian-made Sukhoi Su-27 "Flankers" and Su-30s, along with the domestically built J-10, a state-of-the-art fighter that Beijing just rolled out in January.

China has also improved its ballistic missile defenses and its ability to take the fight into space — as it proved in January by shooting down an old weather satellite at an orbital height similar to that used by the U.S. military.

Wright stressed he is "positive" about the efforts to increase diplomatic and political engagement with Beijing. But he said the Chinese military buildup is disconcerting.

China says spending for its People's Liberation Army, the world's largest standing army with 2.3 million members, grew 17.8 percent this year to nearly $45 billion.

The Pentagon estimates China's actual defense spending may be much higher, because the official budget does not include money for high-priced weapons systems and some other items.

In the U.S., the Senate is wrapping up debate on a $672 billion defense policy bill that would authorize more than a half trillion dollars in annual defense spending and $150 billion for the wars in Iraq and Afghanistan.

"Are we in trouble? It depends on the scenario," Wright said Thursday. "But you have to be concerned about the small number of our forces and the age of our forces."

Wright noted the Air Force's fleet is older than ever before.

The average age of the F-15 fighters, for example, is about 24 years, while that of the KC-135 Stratotanker, a mid-air refueling plane that is a key element in the Air Force's ability to conduct long-range missions, is 46 years.

Wright, who was at this air base on Japan's southern island of Okinawa to meet with local commanding officers, said the improvement in Chinese air defenses has made China's airspace "difficult if not impossible" to penetrate with the kind of U.S. fighters — F-15s and F-16s — now deployed in Japan.

"Our planes are much older than the planes they would be matched against," Wright said. "For the first time in history, we are seeing another nation, in this case China, with newer fighters than we have. We know that they continue to invest at a level that is unprecedented."