Defense Secretary Robert M. Gates denied today that the United States is planning for war with Iran, saying that U.S. military efforts are focused on countering alleged Iranian activities against American troops in Iraq.

In a Pentagon briefing with Marine Gen. Peter Pace, the chairman of the Joint Chiefs of Staff, Gates said the military is putting together details of the Iranian involvement, especially Iran's purported role in supplying sophisticated shaped explosive charges used in roadside bomb attacks on U.S. forces.

Gates also disputed a Congressional Budget Office study that estimates the size and cost of President Bush's planned "surge" of U.S. troops into Iraq at about double the existing figures. And he said the troop strength of the Iraqi forces now deploying as part of a new U.S.-Iraqi security plan for Baghdad is probably insufficient and needs to be increased.

The two officials took issue with the conclusion in a new National Intelligence Estimate that "the term 'civil war' accurately describes key elements of the Iraqi conflict." Gates said the words "oversimplify a very complex situation in Iraq" and stressed that the country does not have "a divided army, a divided government in the sense that I have always thought of a civil war."

Pace said using the term amounts to "putting a bumper sticker" on a highly complicated issue and "really doesn't help solve the problem."

The National Intelligence Estimate said that while the term "does not adequately capture the complexity of the conflict in Iraq," it nevertheless correctly applies to "the hardening of ethno-sectarian identities, a sea change in the character of the violence, ethno-sectarian mobilization and population displacements."

Asked about public suspicions that the Bush administration is gearing up to attack Iran, Gates said, "The president has made clear, the secretary of state has made clear, I've made clear -- nobody is planning -- we are not planning for a war with Iran. What we are trying to do is, in Iraq, counter what the Iranians are doing to our soldiers, their involvement in activities, particularly these explosively-formed projectiles that are killing our troops."

The United States is also trying to get Iran to stop its uranium enrichment program and is doing so "strictly through the diplomatic process," Gates said. He said that effort "seems to be showing some progress; at least the diplomatic process is working."

Gates said the recent deployment of a second U.S. carrier group in the Persian Gulf has fueled speculation of war preparations. However, "the purpose of that is simply to underscore to our friends, as well as to our potential adversaries in the region, that the United States has considered the Persian Gulf and that whole area and stability in that area to be a vital American national interest." He said friendly nations "can count on us having a presence and being strong in their area in protecting our interests and in protecting theirs."

Gates said it is "not clear yet" whether Iranians were involved in a brazen Jan. 20 attack on U.S. soldiers at a provincial center in Karbala, a city 30 miles south of Baghdad that is sacred to Shiite Muslims. Up to a dozen men wearing U.S. Army-style uniforms drove into the compound, fought U.S. troops with rifles and grenades and abducted four American soldiers who were later killed. Another American soldier was killed in the attack.

Asked if he had any indications that Iranians were involved in the planning or execution of the attack, Gates said, "I know there's a lot of speculation about this. I would just tell you flatly that the investigation is still going on, and the information that I've seen is ambiguous."

Pressed for evidence of any Iranian activities resulting in the death of U.S. troops in Iraq, Gates said that "we have seen evidence of Iranian involvement" in providing explosively formed projectiles, known as EFPs, that are used in powerful improvised explosive devices, the roadside bombs referred to by the military as IEDs. He said Iran has provided "either or both the technology and the weapons themselves that have been killing American soldiers."

Gates added, "And so our effort is aimed at uprooting the networks that are providing these EFPs. We're also trying to uproot the networks that provide the IEDs as well that are being provided or being used by al-Qaeda and others." He noted that improvised explosive devices "account for about 70 percent of our casualties."

The U.S. military headquarters in Iraq is putting together "some specifics about the Iranian supply of these weapons," Gates said. But he, Secretary of State Condoleezza Rice and National Security Adviser Stephen J. Hadley have delayed a briefing on the matter because they want to make sure it is "absolutely accurate and is dominated by facts -- serial numbers, technology and so on," Gates said.

He said it remains unclear who in Iran is behind the supply of such weapons, whether it be the Iranian government, militants in the Revolutionary Guards or "rogue elements."

While IED attacks involving explosively formed projectiles probably account for "a relatively small percentage of the overall number of attacks," Gates said, "they are far more lethal."

Pace said the military is "working day and night to disassemble these networks" responsible for the explosives, "and we do that without regard to nationality, but just with regard to who our enemies are." He said it was "instructive that at least twice in the last month that, in going after the networks, we have picked up Iranians."

On Jan. 11, U.S. troops launched two raids on Iranian targets in Iraq, detaining at least five Iranians in the northern city of Irbil and seizing documents and computer data. In December, U.S. forces seized two senior Iranians -- Brig. Gen. Mohsen Chirazi and Col. Abu Amad Davari -- in a first round of raids. Chirazi is the No. 3 official in the al-Quds Brigade of the Revolutionary Guard Corps and the highest ranking Iranian ever held by the United States.

Chirazi and Davari were detained in the Baghdad compound of a leading Iraqi Shiite Muslim politician, Abdul Aziz al-Hakim, and the Iraqi government subsequently demanded their release and expelled them to Iran. U.S. defense officials said at the time that among the sensitive intelligence information in the Iranians' possession were detailed weapons lists, documents pertaining to shipments of weapons into Iraq, organizational charts, telephone records and maps. Officials said the material included information about importing explosively formed projectiles into Iraq, The Washington Post reported in December.

The shaped charges "can take out an Abrams tank," Gates said today.

Pace said the military in Iraq is also looking into the downing of four U.S. helicopters -- three military and one private -- in the past couple of weeks, including one that went down today northwest of Baghdad. Two U.S. soldiers were killed in the latest incident.

"Clearly, there's been more effective ground fire . . . against our helicopters in the last couple of weeks," Pace said. He said it was not known "whether or not this is just statistically what's going to happen over time when you're flying at that level and people are shooting at you, or if there's some kind of new tactics and techniques that we need to adjust to."

Asked to respond to a comment yesterday by the top U.S. commander in Iraq that the first Iraqi units designated for the new Baghdad security plan are now at 55 percent to 65 percent strength, Gates said it was not yet clear whether that meant the Iraqi government was not meeting its commitments.

"I guess my answer is 55 percent probably isn't good enough, but I'm not sure what the end strength of that unit is going to be when the time comes for it to go into combat," Gates said. He said Iraqi troops are paid in cash and often go home to deliver their pay to their families, "and so there is an absentee level that doesn't represent desertion."

Pace said there was "good news and bad news" in the Iraqi deployment. "The good news is, is that, contrary to what has happened in the past, the units that were designated to arrive in Baghdad have begun to arrive" on schedule. However, he said, "the initial units got there with about 60 percent, and therefore they do need to continue to flesh out those units."

Gates also sought to explain a Congressional Budget Office study, disclosed yesterday in a letter to the chairman of the House Budget Committee. It said that when support troops are added to the 21,500 soldiers and Marines being sent to Iraq as part of Bush's surge plan, the total number of extra personnel could range from about 35,000 to 48,000, with costs ranging from $9 billion to $13 billion for a four-month deployment to $20 billion to $27 billion for a 12-month deployment.

Gates said the CBO study "dramatically overstates both the cost and the personnel." In part, he said, this is because the Pentagon's cost estimate goes through September of this year, or the end of fiscal 2007, while "the CBO number goes out to the end of FY '09."

In addition, the CBO's estimate for support personnel "is dramatically higher" than the military's, he said. He estimated that it would end up being "about 10 percent to 15 percent of the number that CBO cited."