But the Pentagon insists that it is not encroaching on the CIA's turf and says all its activities are permissible under existing laws and executive orders.

In some cases, the clandestine operations involve inserting U.S. military personnel in countries unaware of the intrusion. Officials emphasized that the military has previously executed such delicate missions, but never before on such a large scale.

"The volume of these smaller-scale clandestine activities has expanded dramatically," said the Defense official.

Pentagon officials declined to provide details about specific operations or discuss countries where clandestine activities are underway. But their descriptions make it clear that the Pentagon is seeking to improve its ability to gather intelligence within the borders of such countries as Iran, North Korea and China.

"Our ability to collect inside the national territories of these potential adversaries — that is a challenge to us," said another Defense official. "There's no silver bullet here."

Defense officials say they have been granted no new authority since the Sept. 11 attacks to carry out "covert" operations — missions that the U.S. government can deny knowing about and that require presidential authority. Covert operations are designed to influence the political, economic or military conditions within another country's borders, and traditionally are carried out by CIA operatives.

At the same time, the Pentagon is using a broad definition of its current authority to conduct what it describes instead as "clandestine" operations around the globe — dispatching military teams to gather intelligence about potential adversaries. Unlike covert operations, clandestine missions are not intended to influence the internal dynamics of another nation, according to U.S. officials.

"If we're getting information, then the last thing we want to do is influence the country, because then we're detectable," said the first Defense official.

Using that definition, Defense and congressional officials said military personnel could enter other countries to gather intelligence without getting advance approval from the president or giving notice to Congress.

Since the Sept. 11 attacks, Defense Secretary Donald H. Rumsfeld has chafed at the Defense Department's reliance on CIA spies to provide on-the-ground intelligence to U.S. military commanders.

Rumsfeld ordered an overhaul and upgrade of the Pentagon's intelligence apparatus in 2003, and Wednesday four Defense officials involved in the restructuring discussed some of the results of the effort in interviews with The Times.

The changes outlined by the officials lay out the significantly expanded espionage role for the U.S. military. Former officials said there was friction between the Pentagon and the CIA.

"This is a turf battle," said retired Army Col. W. Patrick Lang, former head of Middle Eastern affairs for the Defense Intelligence Agency. "All of this represents that clandestine human intelligence in the Department of Defense is a growth industry and that it is no longer regarding itself as under the control of the CIA."

Defense officials, in interviews Wednesday, outlined an expansion of the military's intelligence-gathering capabilities across an array of fronts, from low-level soldiers canvassing neighborhoods in Baghdad to highly trained Defense Department "case officers" working in undercover assignments overseas.

Some senior military officials are concerned about increased emphasis on espionage, fearing that soldiers caught while carrying out clandestine operations might lose the protections accorded under international law for captured military personnel.

Unlike CIA operatives, U.S. troops enjoy Geneva Convention protections and their activities are traditionally acknowledged by the U.S. government.

One Defense official said that these traditional lines may have blurred, and that in some cases, Washington might not acknowledge the identity of a soldier or civilian captured during an intelligence-gathering mission.

"The decision about whether to reveal the affiliation [of that individual] is something that would be handled on a case-by-case basis," he said.

The Pentagon is planning to increase the authority of military intelligence officers within each geographic combatant command. Each command will have a general or admiral directing an intelligence apparatus known as a Joint Intelligence and Operation Command, or JIOC, charged with gathering and analyzing intelligence collected in that theater of operations.

The first JIOC will be established inside Central Command, based in Tampa, Fla., which has military authority over the Middle East and Central Asia.

Under the new plan, the senior intelligence officer within each combatant command could be given authority equivalent to that of the senior Army, Navy, Air Force, Marine and special operations commanders.

Much of the Pentagon's new intelligence activity is centered in the Defense Intelligence Agency, the military's main spying service. Under a program called the Strategic Support Branch, the DIA is assembling teams of interrogators, analysts and other intelligence operatives that are being deployed with special forces units on operations around the globe.

A DIA official said the teams are to be based in the United States but will have expertise in the language, issues and customs of the region to which they are likely to be deployed.

The DIA is also in the midst of a major expansion of its "Defense Humint Service," the military's equivalent of the CIA's overseas spying branch. DHS spies are trained alongside CIA case officers at "the Farm," the CIA's training center in southern Virginia.

The number of DHS case officers, which hovered around 100 in the late 1990s, has multiplied in recent years. The DIA official declined to say how many officers are part of the service now, but noted that the agency added 1,200 positions across all job categories last year, and expects to add an additional 600 to 800 this year.

Former military and intelligence officials said the DIA's mission was also expanding. Military operatives have long studied other nations' militaries and conducted surveillance of landing zones and bridges where U.S. forces might be inserted.

"But DIA is now engaged in doing far grander things with regard to trying to penetrate foreign organizations," said Lang, the former DIA official. "They're trying to penetrate jihadi organizations and they're doing battlefield reconnaissance in preparation for special operations in various places.

"It's happening all over the Islamic world."

A congressional official familiar with the military's clandestine activities said the Pentagon had "gotten much more aggressive in intelligence collection in a variety of areas" over the last two years.

The congressional official also said that though intelligence-gathering operations by the Pentagon weren't considered "covert action," the military nevertheless was often required to provide advance notice to Congress.

In particular, sections of the 1947 National Security Act require the military or other agencies to notify lawmakers in advance of "significant" intelligence activities. That is generally interpreted to mean operations in which operatives risk being captured or killed. It also applies to missions that could damage U.S. foreign policy if they were uncovered.

Under that definition, almost any operation inside Iran "would qualify as a 'significant' intelligence activity," the congressional official said.

He declined to say whether the intelligence committees had received notification of such an operation.