By Damien McElroy
Karl Eikenberry, a former commanding general in Afghanistan, said parts of the regime had transcended sectarian divisions within Islam to provide support for fundamentalist groups fighting Western forces in Afghanistan.
"Iran or elements within Iran have provided training assistance and some weapons to the Taliban," said Mr Eikenberry.
"General Petraeus has reviewed these reports and said that the scope of Iranian support is nothing on the level that was given previously by Iran to various terrorist elements in Iraq.
"Still, the reports about this kind of low-level support and periodic co-operation between elements in Iran and militant extremist Taliban are disturbing and do not show good faith by Afghanistan's neighbour to the West."
Iran's Shia Muslim regime's has long been suspicious of the extremist Sunni Taliban and Tehran co-operated with the US-led effort to overthrow the movement in 2001.
But a Western official involved in Iran policy-making said yesterday that Iranian officials were now playing both sides of the Afghan conflict to ensure that the Western-backed Kabul government remained weak.
"Afghanistan should be an area of common interest between Britain and Iran because they don't want an extremist Sunni government on their border," an official said.
"But Afghanistan, like Iraq, is in its backyard and Tehran just does not want a Western victory or a strong pro-Western government on its eastern flank. It is now playing two roles there, assisting the insurgency even as it provides aid."