Taiwan's Defense Ministry said it arrested a military general on suspicion of spying for China in the most high-profile cross-Strait espionage case in decades.

Taiwan government officials and some experts said the case highlights a determined effort by China to infiltrate the island's military despite warming economic and political ties between the two sides.

The Ministry of National Defense confirmed on Wednesday it arrested Maj. Gen. General Lo Hsien-che on suspicion of leaking confidential information to Chinese intelligence sources after he was approached by Chinese operatives in 2004 while he worked in Thailand as a military attache.

Although it remained uncertain just what information Gen. Lo might have leaked, the arrest could complicate further U.S. military sales to Taiwan, according to Chih-cheng Lo, president of the Taiwan Brain Trust, a think-tank that advocates independence for the island, which China claims as its own.

"China has been very aggressive lately in its efforts to penetrate Taiwan's military," Mr. Lo said. "This isn't an isolated case, there are most likely more," he said. "It's likely now we're in a time when the U.S. is thinking about sending F-16 C/D fighters. If Taiwan can't rectify some of these problems, the U.S. may reconsider some of its lines of exchange with Taiwan."

Local media, citing military sources, reported that General Lo had access to secret documents on the Po Sheng program, a system that integrates ground, naval, and air forces with command centers being sold to Taiwan by U.S. contractor Lockheed Martin Corp. as well as classified documents related to the Apache helicopter procurement plan.

But Jung-feng Chang, former deputy of Taiwan's National Security Council, said he doubted the reports. "The U.S. has very strict anti-espionage regulations, and I would be very surprised if Lo were able to get his hands on any critical information," he said.

In a press conference on Wednesday morning, Lt. Gen. Wang Ming-wo of the Defense Ministry's Political Warfare Bureau said the military had established a team to control and limit any damage caused by the leaks. He said that despite warming ties with China, the country's efforts to infiltrate Taiwan have continued in a "smokeless war."

In recent years, following the election of the China-friendly Kuomintang party and President Ma Ying-jeou, cross-Strait relations have thawed on the back of a series of economic agreements that have liberalized trade and investment. But despite the warming ties, China maintains that Taiwan is a part of its territory and has threatened in the past to use military action to claim the island.

Mr. Lo of the Taiwan Brain Trust said that as Taiwan and China grow closer, the risk of further infiltration by China into Taiwan's military and government would increase.

Under the 1979 Taiwan Relations Act, the U.S. is obligated to sell weapons to Taiwan for its defense. But Washington has yet to approve sales of some advanced weapons systems that Taiwan's military has requested, including Taiwan F-16 C/D fighters.