WASHINGTON – American troops in Iraq have authority to kill or capture Iranian agents deemed to be a threat, officials said Friday in describing a tougher stand toward Tehran and its suspected meddling in the nearly four-year-old war.

The more aggressive policy – evolving over a period of months – was described as the result of mounting evidence that Iran is supporting terrorists inside Iraq and is a major supplier of bombs and other weapons used to target U.S. forces.

President Bush has said the remote-controlled bombs, called improvised explosive devices, are the greatest threat to U.S. troops and the future of a peaceful Iraq.

“Our forces are authorized to go after those who are trying to kill them,” Defense Secretary Robert Gates said. “And we are trying to uproot these networks that are planting IEDs that are causing 70 percent of our casualties. And if you’re in Iraq and trying to kill our troops, then you should consider yourself a target.”

The United States and Iran have regarded each other with distrust and suspicion since the 1979 takeover of the U.S. Embassy in Tehran by militant students. Most recently, tensions have flared over U.S.-led efforts to isolate Iran and force it to abandon its suspected nuclear weapons program.

National Security Council spokesman Gordon Johndroe said the president and his national security team have received a stream of information over the last several months pointing toward Iranian involvement.

“As a result, American forces – when they receive actionable information – may take the steps necessary to protect themselves as well as the population,” Johndroe said.

Bush said he made it clear to Army Lt. Gen. David Petraeus, the new commander of U.S. forces in Iraq, that “our policy is going to be to protect our troops in Iraq.

“It makes sense that if somebody is trying to harm our troops or stop us from achieving our goal, or killing innocent citizens in Iraq, that we will stop them,” Bush said in an Oval Office meeting with Petraeus, Gates and Gen. Peter Pace, chairman of the Joint Chiefs of Staff.

While promising tougher action, the White House said the United States does not intend to cross the Iraq-Iran border to attack Iranians.

Bush, at his appearance with Petraeus, said the tougher policy did not mean that “we want to expand this beyond the borders” of Iraq. “That’s a presumption that simply is not accurate.”

“We believe that we can solve our problems with Iran diplomatically and are working to do that,” the president said. “As a matter of fact, we’re making pretty good progress on that front.”

Still, the aircraft carrier USS John C. Stennis and accompanying warships have been sent to the Persian Gulf as part of a buildup of forces that Gates has said is aimed to impress on Iran that the Iraq war has not made America vulnerable.