Senior U.S. defense officials say they have evidence linking some of the deadliest weapons being used against U.S. forces in Iraq to the highest levels of the Iranian government, but congressional Democrats are warning the administration to go slow in making its case.

"Every leader in the region and every observer, every expert here in our country, tells us that Iran does not want a complete and total implosion in Iraq," Massachusetts Sen. John Kerry said Sunday.

According to U.S. military officials, a significant increase in a number of explosively formed projectiles, or EFPs, found in Iraq have tracked back to Iran. EFPs, which can penetrate heavy armor used in tanks, have killed 170 American troops since 2004.

According to the officials in Baghdad, they recently confiscated a number of EFPS that were found before exploding. The C-4 explosive in them has been chemically traced to Iran, the "machining process" required to make the projectile is not available in Iraq and the triggering devices are also traceable to Iran, they said.

That is also true of a significant number of mortar rounds as well as rocket-propelled grenades, which can be identified by the markings and designs on the tailfin of the mortars, the officials said. The date on most of the ammunition is 2006, which means it was manufactured in 2006 and is not material left over from the era of Saddam Hussein. The officials said the Iranian supplies are mostly going to surrogates here, primarily Mahdi Army militia members.

The Mahdi militia is faithful to Shiite cleric Muqtada al-Sadr.

Officials said they developed the information in part on the arrest of the five operatives in Irbil, including Mohsin Chizari, an operation chief in the Iranian Revolutionary Guard, an elite arm of the Quds force that reports to Iran's supreme leader.

The conclusions are also based on documents seized during a December raid on the Hakim compound in Baghdad. In the raid, troops found inventory sheets of weapons supplied. U.S. officials said they are announcing this information in order to put Iran on notice publicly.

But Democratic Sen. Jack Reed wondered whether the influx of Iranian armaments was a plan by the Islamic regime in Tehran or just "rogue elements" within it.

"There are certainly indications, as Mr. Gates pointed out this week, that these explosive foreign projectiles seem to be coming from Iran. They've been used. The question is is this a deliberate policy of the Iranian government at the highest levels. Is it rogue elements within the government?" Reed said on "FOX News Sunday," referencing Defense Secretary Robert Gates.

"And then the other question is to what extent are there countervailing signals that the Iranians actually are trying to — not control, but not to further raise the stakes in Iraq," he said.

Sen. Christopher Dodd, D-Conn., a 2008 presidential candidate, said the administration could be laying the groundwork for an attack on Iran and that "I'm worried about that. That's how we got into the mess in Iraq," by relying on what Dodd called "doctored information."

Senate Intelligence Committee member Ron Wyden, D-Ore., said, "the administration is engaged in a drumbeat with Iran that is much like the drumbeat that they did with Iraq. We're going to insist on accountability."

Kerry, the 2004 presidential candidate, said despite the evidence, the United States must try to engage Iran diplomatically.

"Ultimately, they want an Iraq that is stable. They want influence. They want to be players in the region. And we need to recognize that and engage in a kind of diplomacy that the Iraq Study Group recommended," Kerry told ABC's "This Week."

Iran's response to the allegations appeared in an op-ed published last week in The New York Times. Even before the military briefing in Baghdad, Iran's spokesman for its mission to the United Nations said America is forging imaginary threats with Iran to provide "temporary domestic cover" for the administrations failure in Iraq.

"Now the United States administration is — unfortunately — reaping the expected bitter fruits of its ill-conceived adventurism, taking the region and the world with it to the brink of further hostility. But rather than face these unpleasant facts, the United States administration is trying to sell an escalated version of the same failed policy. It does this by trying to make Iran its scapegoat and fabricating evidence of Iranian activities in Iraq," wrote Amb. Javad Zarif.

At home, on the Republican side of the aisle, Sen. Trent Lott of Mississippi said he does not think the United States is trying to make a case for attacking Iran. Lott said the U.S. should try to stop the flow of munitions through Iran to Iraq but that "you do that by interdiction ... you don't do it by invasion."

Senate Minority Leader Mitch McConnell, R-Ky., said he has not been briefed fully on the topic, but he's not prepared to say whether the evidence would require U.S. forces crossing Iran's border.

"If we're going to protect our forces in Iraq, and if there are Iranians in Iraq, inside Iraq, seeking to do harm to our soldiers, of course we'll take the appropriate action," he said. "There is not indication that any of this has to do with going beyond Iraq, but inside Iraq, if there are foreigners in there, seeking to harm American soldiers, certainly we are going to respond to that."

Sen. John Cornyn, R-Texas, however, spoke in more aggressive tones, saying that "we have to do everything within our power to stop" any cross-border flow.

FOX News' Rudi Bakhtiar and John Fiegener and The Associated Press contributed to this report.