U.S. and British troops are being killed in Iraq by increasingly sophisticated insurgent bombs, including a new type triggered when a vehicle crosses an infrared beam and is blasted by armor-piercing projectiles.

The technology, which emerged during guerrilla wars in Lebanon and Northern Ireland, has been used in recent roadside bombings that have killed dozens of Americans and at least eight British soldiers.

The alarming efficiency has led many British and a few U.S. officials to argue that rogue groups in Iran and perhaps Lebanon are giving expertise to Iraq's insurgents. But others caution against that idea, saying the technology is available to those who know where to look.

Either way, the Pentagon is scrambling to find countermeasures, says Maj. Gen. Rick Lynch, a senior U.S. military officer in Iraq.

"We're studying very hard where this technology is coming from and what we can do to combat that technology," Lynch said in a briefing in Baghdad last week.

The deadly munitions mark a steady improvement in the roadside bombs that debuted in 2003 in Iraq, often as simple as a single artillery shell wrapped with detonator cord linked to a battery.

The new bombs are a deadly marriage of stealthy camouflage, shaped explosives that propel metal projectiles through four inches of armor and infrared motion-detector triggers that can't be blocked by electronic jammers.

"It works like a burglar alarm, a beam that goes across a doorway. Once the beam is broken it triggers the bomb," said Amyas Godfrey, a former British army intelligence officer who left Iraq in October 2004 after serving two tours.

British officials and, to a lesser extent, their American counterparts have suggested Iraqi insurgents are getting advice and perhaps components from Iran or Lebanon's Hezbollah militia.

In the 1990s, Hezbollah's Iranian-backed Shiite fighters used infrared-triggered penetrator bombs with great success against Israeli armored vehicles in southern Lebanon.

Similar bombs have killed eight British soldiers in southern Iraq since May, and British Prime Minister Tony Blair said the circumstances "lead us either to Iranian elements or to Hezbollah." He conceded he had no evidence, and both Iran and Hezbollah denied involvement.

Anthony Cordesman, a military analyst at the Center for Strategic and International Studies in Washington, said Iraq's insurgents are more likely just tapping a pool of common bomb-making technology, none of which requires special expertise.

"There's no evidence that these are supplied by Iran," he said. "A lot of this is just technology that is leaked into an informal network. What works in one country gets known elsewhere."

Last month, the London-based Independent newspaper quoted a British intelligence official as saying the Irish Republican Army was first to use infrared triggers in bombs aimed at British troops 15 years ago.

The ballistics technology behind the bombs' shaped charges dates to World War II anti-tank munitions.

The insurgent variety uses a cone-shaped plastic explosive charge that concentrates its force on a steel or copper projectile. The projectile is fired at high velocity and stretched into a molten slug that can burn through four inches of armor, Cordesman said.

Infrared triggers are easily obtained, said Godfrey, the former British intelligence officer. He said they are identical to motion sensors used to open elevator doors or set off burglar alarms.

The new bombs also contain simple radio-controlled receivers that allow insurgents to arm them by radio or cell phone ahead of an approaching military convoy.

"Usually they'll place an array of explosives locked to a single infrared sensor," Cordesman said. "What you get is an array of shaped charges, so you're not going to get hit with just one."

He said the clustered projectiles are accurate — and effective — against armored Humvees and light armored vehicles at up to 50 feet. Heavily armored Abrams tanks and Bradley fighting vehicles are better able to withstand the blasts, although some have been destroyed.

Even if a blast doesn't penetrate a vehicle's armor, "the impact will blow off shards of armor inside the vehicle that are red hot and cut people to ribbons," said Bruce Jones, a London-based intelligence expert who advises NATO.

Perhaps most worrisome for the Pentagon is that infrared triggers cannot be blocked by electronic countermeasures, such as devices that emit a radio beam to jam signals from cell phones, garage-door openers and other remote-control devices used to detonate bombs.

"I don't know if you can disrupt an infrared beam without triggering the explosion," said Godfrey, now an analyst for the Royal United Services Institute for Defense and Security Studies.

Researchers are studying whether expensive vision equipment might let soldiers see the beams. Others suggest developing explosive detectors to scan roadsides for bombs or an electromagnetic pulse weapon that could fry unshielded electronics within reach of a convoy.

As the Pentagon searches for a solution, Godfrey said U.S. and British forces are adjusting patrol routes and scanning maps for likely ambush points. Troops keep watch for bombs hidden in hollowed-out trees, the dirt or plastic foam painted to resemble concrete.

"We can get very excited about covering ourselves with technology. But at the end of the day, you have to think like an insurgent," Godfrey said.