NATO officials say they have caught Iran red-handed, shipping heavy arms, C4 explosives and advanced roadside bombs to the Taliban for use against NATO forces, in what the officials say is a dramatic escalation of Iran's proxy war against the United States and Great Britain.

"It is inconceivable that it is anyone other than the Iranian government that's doing it," said former White House counterterrorism official Richard Clarke, an ABC News consultant.

Secretary of Defense Robert Gates stopped short earlier this week of blaming Iran, saying the U.S. did not have evidence "of the involvement of the Iranian government in support of the Taliban."

But an analysis by a senior coalition official, obtained by the Blotter on ABCNews.com, concludes there is clear evidence of Iran's involvement.

"This is part of a considered policy," says the analysis, "rather than the result of low-level corruption and weapons smuggling."

Iran and the Taliban had been fierce enemies when the Taliban was in power in Afghanistan, and their apparent collaboration came as a surprise to some in the intelligence community.

"I think their goal is to make it very clear that Iran has the capability to make life worse for the United States on a variety of fronts," said Seth Jones of the Rand Institute, "even if they have to do some business with a group that has historically been their enemy."

The coalition analysis says munitions recovered in two Iranian convoys, on April 11 and May 3, had "clear indications that they originated in Iran. Some were identical to Iranian supplied goods previously discovered in Iraq."

The April convoy was tracked from Iran into Helmand province and led a fierce firefight that destroyed one vehicle, according to the official analysis. A second vehicle was reportedly found to contain small arms ammunition, mortar rounds and more than 650 pounds of C4 demolition charges.

A second convoy of two vehicles was spotted on May 3 and led to the capture of five occupants and the seizure of RPG-7mm rockets and more than 1,000 pounds of C4, the analysis says.

Also among the munitions are components for the lethal EFPs, or explosive formed projectiles, the roadside bombs that U.S. officials say Iran has provided to Iraqi insurgents with deadly results.

"These clearly have the hallmarks of the Iranian Revolution Guards' Quds force," said Jones.

The coalition diplomatic message says the demolition charges "contained the same fake U.S. markings found on explosives recovered from insurgents operating in the Baghdad area."

"We believe these intercepted munitions are part of a much bigger flow of support from Iran to the Taliban," the message says.

The Taliban receives larger supplies of weapons through profits from opium dealing, officials say, but the Iranian presence could be significant.

"It means the insurgency in Afghanistan is likely to be prolonged," said Jones. "It would be a much more potent force." ABC News