U.S. officials tell CBS News that American forces have begun an aggressive and mostly secret ground campaign against networks of Iranians that had been operating with virtual impunity inside Iraq.
The chairman of the Joint Chiefs of Staff told Congress on Friday that Iranians are now on the target list.
"Twice in the last two or three weeks, in pursuit of those networks, when we have gone and captured those cells, we've captured Iranians," said Gen. Peter Pace.
According to U.S. military figures, 198 American and British soldiers have been killed, and more than 600 wounded by advanced explosive devices manufactured in Iran and smuggled in through the southern marshes and along the Tigris River. Attempts to disrupt these networks, combined with the decision to send a second aircraft carrier to the Persian Gulf as a warning to Iran, significantly raises the stakes, according to former Assistant Secretary of State Martin Indyk.
"It's going to have, you would expect, some rather serious consequences," he said.
Defense Secretary Robert Gates tried to assure the Senate Armed Services Committee the U.S. military will not widen the war into Iran.
"We believe that we can interrupt these networks that are providing support through actions inside the territory of Iraq and there is no need to attack targets in Iran itself," he said.
But with the future of oil-rich Iraq at stake, the revolutionary leaders of Iran are not likely to back down.
"Since the president has taken the gloves off, I would expect that they would respond by taking the gloves off, too," Indyk says.
Pentagon sources tell CBS News the U.S. military has planned covert cross-border raids into Iran — but so far none has been approved.
Meanwhile, Sen. John McCain defended President Bush's Iraq plan on Friday as a difficult but necessary move, parting company with lawmakers fiercely resisting the military build up.
"I believe that together these moves will give the Iraqis and Americans the best chance of success," said McCain, R-Ariz., a leading presidential contender for 2008.
McCain also took a shot at Democrats who say the U.S. must bring home some troops within four to six months.
"I believe these individuals ... have a responsibility to tell us what they believe are the consequences of withdrawal in Iraq," he said. "If we walk away from Iraq, we'll be back, possibly in the context of a wider war in the world's most volatile region."
McCain spoke at the Senate Armed Services Committee, where Gates and Gen. Peter Pace, chairman of the Joint Chiefs of Staff, spent a second day on Capitol Hill defending the president's strategy.
As they did, Democrats continued considering strategies for challenging Mr. Bush's war policies. One influential lawmaker, Rep. John Murtha, D-Pa., said he'd like to require closing the U.S. prison at Guantanamo Bay, Cuba, and better training for troops heading to the war zone as conditions of Congress providing more money for Iraq.
"We have to close the prison at Guantanamo," said Murtha, who heads the House panel that controls the Pentagon's budget. He said Democrats would decide later whether to pursue the idea.
The Bush administration has said military the detention center is still needed. It holds almost 400 detainees suspected of links to al Qaeda and the Taliban.
Gates and Pace on Friday assured lawmakers there were no immediate plans to attack targets in Iran. In , Mr. Bush vowed to disrupt Iran's aid to insurgents in Iraq and "destroy the networks providing advanced weaponry and training to our enemies in Iraq."
Despite pointed questions from Democrats, the testimony of the two top officials drew considerably less consternation than Thursday's testimony from Secretary of State Condoleezza Rice. Sen. Chuck Hagel, R-Neb., told Rice that he feared Mr. Bush's plan would be the worst foreign policy blunder since the Vietnam War.
On the Senate Armed Services Committee are several staunch Bush supporters, including John Cornyn of Texas, Jeff Sessions of Alabama and Saxby Chambliss of Georgia. In addition to McCain, committee members Lindsey Graham, R-S.C., and Joseph Lieberman, I-Conn., support sending more troops to Iraq.
Sen. Carl Levin, D-Mich., the panel's new chairman, said deepening America's commitment in Iraq would be a grave mistake. Mr. Bush wants to add 21,500 more U.S. troops to the 132,000 already there.
"Increasing the number of U.S. forces in Iraq is flawed strategy because it is based on a flawed premise that there is a military solution to the violence and instability in Iraq, when what is needed is a political solution among the Iraqi leaders and factions," Levin said.
Repeating an admission that the president made in his nationally televised address on Wednesday, Gates told the senators, "Mistakes certainly have been made by the United States in Iraq. However we got to this moment, the stakes now are incalculable."
Mr. Bush on Friday sought support for his new Iraq military buildup in telephone calls to Jordan's King Abdullah II and Egyptian President Hosni Mubarak.
Late Thursday, the bipartisan Iraq Study Group, led by James Baker and Lee Hamilton, warned against sending more troops for long. The group had called for withdrawal of U.S. combat troops by early 2008, but said that a temporary troop increase might be justified under some circumstances.
"We are encouraged by the president's statement that 'America's commitment is not open-ended' and Secretary Gates' statement that the addition of 21,000 troops would be viewed as a temporary surge," Baker and Hamilton said in a statement. "The violence in Baghdad will not end without national reconciliation."
Republican Sens. John Warner and Susan Collins said Friday they were gravely concerned about the fate of Iraq. Collins, R-Maine, asked Gates and Pace why the administration thinks the plan will work when past attempts have failed.
Warner said the goal must be to keep Iraq from imploding and being "scattered to the winds" in the region.
"I don't call it victory. I don't call it a win," said Warner, R-Va. "But to enable this government and its people to continue to seek their own level of democracy and freedom."
On Thursday, Gates announced that he was requesting an increase in the size of the Army and Marine Corps by 92,000 troops over the next five years.
He also said the Pentagon would recall to duty sooner than planned who have served yearlong tours in Iraq or Afghanistan.