WASHINGTON - The Bush administration has authorized the U.S. military to kill or capture Iranian operatives inside Iraq as part of an aggressive new strategy to weaken Iran's influence across the Middle East and compel it to give up its nuclear program, according to government and counterterrorism officials with direct knowledge of the effort.

For more than a year, U.S. forces in Iraq have secretly detained dozens of suspected Iranian agents, holding them for three to four days at a time. The "catch-and-release" policy was designed to avoid escalating tensions with Iran and yet intimidate its emissaries. U.S. forces collected DNA samples from some of the Iranians without their knowledge, subjected others to retina scans, and fingerprinted and photographed all of them before letting them go.

Last summer, however, senior administration officials decided a more confrontational approach was necessary, as Iran's regional influence grew and U.S. efforts to isolate the country appeared to be failing. The country's nuclear work was advancing, U.S. allies were resisting robust sanctions against the Tehran government, and Iran was aggravating sectarian violence in Iraq.

"There were no costs for the Iranians," one senior administration official said. "They are hurting our mission in Iraq, and we were bending over backwards not to fight back."

Three officials said that about 150 Iranian intelligence officers, plus members of Iran's Revolutionary Guard Command, are believed to be active inside Iraq at any given time. There is no evidence the Iranians have directly attacked U.S. troops in Iraq, intelligence officials said.

But, for three years, the Iranians have operated an embedding program there, offering operational training, intelligence and weaponry to several Shiite militias connected to the Iraqi government, to the insurgency and to the violence against Sunni factions. Gen. Michael Hayden, the director of the CIA, told the Senate recently that the amount of Iranian-supplied materiel used against U.S. troops in Iraq "has been quite striking."

The new "kill-or-capture" program was authorized by President Bush in a meeting of his most senior advisers last fall, officials said, along with other measures meant to curtail Iranian influence from Kabul to Beirut and, ultimately, to shake Iran's commitment to its nuclear efforts. Iran insists its nuclear program is peaceful, but the United states and other nations say it is aimed at developing weapons.