The war of words between the US and Iran has spilled into Afghanistan after US allegations that Iran is secretly supporting the Taliban insurgency.
The charges, expressed in carefully calibrated language, represent the first time senior US officials have publicly aired rumours that have circulated privately in Afghanistan since last year. Iran denied the charges, and sceptical western officials noted they coincided with mounting speculation about possible US air strikes against Iranian nuclear facilities.
On Monday General Peter Pace, chairman of the Joint Chiefs of Staff, said coalition forces in southern Kandahar province had confiscated a large shipment of Iranian-made mortars and plastic explosives destined for the Taliban. He said it was unclear "which Iranian entity" had shipped the arms.
Yesterday his comments were echoed by the assistant secretary of state for south Asia, Richard Boucher, who said there were signs that Iran was involved in "unhealthy" activities, including contacts with and supplies to the Taliban.
"We don't know exactly who is doing this and why but we know that these are Iranian-origin weapons that have shown up in the hands of the Taliban," he said.
A senior Afghan general told the Guardian he also had evidence of Iranian meddling. Brigadier General Mohiyadeen Ghori, commander of the 205th Corps stationed in Helmand, said his troops had recovered Iranian-manufactured weapons from insurgents in Farah province, and that Iran was funding insurgents in Garmser district, where several British soldiers have died in heavy fighting.
The funds were being channelled through drug smugglers based in Iranian Baluchistan, said the general. "All they are interested in is money," he said.
Iran's ambassador to Kabul, Mohammad Reza Bahrami, denied the accusations. "This news comes to me with surprise and regret ... our borders are frontiers of friendship and peace," he said, adding that Iran has donated $250m (£125m) to Afghanistan since 2001. "We hope this will not be a justification for the actions of foreign forces."
Discerning the exact nature of Iran's relationship with Afghanistan is a complex task. Even US officials admit Tehran has been a staunch supporter of President Hamid Karzai and played a positive role in combating drug smuggling.
But Mr Karzai's close relationship with Washington makes Tehran uncomfortable, as does the presence of more than 20,000 US troops in Afghanistan, and since last year rumours of covert Iranian support for the Taliban have circulated among some diplomats and security officials. Such an alliance is possible despite tensions between Sunni-dominated Taliban and Shia-dominated Iran, said Wadir Safi of Kabul university. "This is political Islam and they have one common enemy: America," he said.
Some western officials in Kabul treated the allegations of Taliban support with scepticism yesterday. One said Iranian weapons had been freely available on the black market since the 1990s, when Tehran shipped arms to groups fighting the Taliban. "Out of every 10 Kalashnikovs, one is Iranian," he said. "This is all a war of words. It has very little basis in reality."