Despite a six-year effort to build trusted computer chips for military systems, the Pentagon now manufactures in secure facilities run by American companies only about 2 percent of the more than $3.5 billion of integrated circuits bought annually for use in military gear.

That shortfall is viewed with concern by current and former United States military and intelligence agency executives who argue that the menace of so-called Trojan horses hidden in equipment circuitry is among the most severe threats the nation faces in the event of a war in which communications and weaponry rely on computer technology.

As advanced systems like aircraft, missiles and radars have become dependent on their computing capabilities, the specter of subversion causing weapons to fail in times of crisis, or secretly corrupting crucial data, has come to haunt military planners. The problem has grown more severe as most American semiconductor manufacturing plants have moved offshore.

Only one-fifth of all computer chips are now made in the United States, and just one-quarter of the chips based on the most advanced technologies are built here, I.B.M. executives say. That has led the Pentagon and the National Security Agency to expand significantly the number of American plants authorized to manufacture chips for the Pentagon’s Trusted Foundry program.

Despite the increases, semiconductor industry executives and Pentagon officials say, the United States lacks the ability to fulfill the capacity requirements needed to manufacture computer chips for classified systems.

“The department is aware that there are risks to using commercial technology in general and that there are greater risks to using globally sourced technology,” said Robert Lentz, who before his retirement last month was in charge of the Trusted Foundry program as the deputy assistant defense secretary for cyber, identity and information assurance.

Counterfeit computer hardware, largely manufactured in Asian factories, is viewed as a significant problem by private corporations and military planners. A recent White House review noted that there had been several “unambiguous, deliberate subversions” of computer hardware.

“These are not hypothetical threats,” the report’s author, Melissa Hathaway, said in an e-mail message. “We have witnessed countless intrusions that have allowed criminals to steal hundreds of millions of dollars and allowed nation-states and others to steal intellectual property and sensitive military information.”

Ms. Hathaway declined to offer specifics.

Cyberwarfare analysts argue that while most computer security efforts have until now been focused on software, tampering with hardware circuitry may ultimately be an equally dangerous threat. That is because modern computer chips routinely comprise hundreds of millions, or even billions, of transistors. The increasing complexity means that subtle modifications in manufacturing or in the design of chips will be virtually impossible to detect.

“Compromised hardware is, almost literally, a time bomb, because the corruption occurs well before the attack,” Wesley K. Clark, a retired Army general, wrote in an article in Foreign Affairs magazine that warns of the risks the nation faces from insecure computer hardware.

“Maliciously tampered integrated circuits cannot be patched,” General Clark wrote. “They are the ultimate sleeper cell.”

Indeed, in cyberwarfare, the most ancient strategy is also the most modern.

Internet software programs known as Trojan horses have become a tool of choice for computer criminals who sneak malicious software into computers by putting it in seemingly innocuous programs. They then pilfer information and transform Internet-connected PCs into slave machines. With hardware, the strategy is an even more subtle form of sabotage, building a chip with a hidden flaw or a means for adversaries to make it crash when wanted.

Pentagon executives defend the manufacturing strategy, which is largely based on a 10-year contract with a secure I.B.M. chipmaking plant in Burlington, Vt., reported to be valued as high as $600 million, and a certification process that has been extended to 28 American chipmakers and related technology firms.

“The department has a comprehensive risk-management strategy that addresses a variety of risks in different ways,” said Mitchell Komaroff, the director of a Pentagon program intended to develop a strategy to minimize national security risks in the face of the computer industry’s globalization.

Mr. Komaroff pointed to advanced chip technologies that made it possible to buy standard hardware components that could be securely programmed after they were acquired.