Reports from Afghan spies and paid informants, described in papers published on whistleblower website Wikileaks, accuse the Iranian government of directly supporting the insurgents.
These "threat reports" cannot be corroborated, the Guardian newspaper said in a report summarizing the Iran findings, but high-level US diplomatic communications indicate concern over Iran's growing involvement in the country.
"Iran has taken a series of steps to expand and deepen its influence in Afghanistan," reads a summary of a secret cable sourced to the US embassy in Kabul and written by a deputy general.
The cable relayed claims from within the Afghan foreign ministry that Iran was bribing Afghan MPs with millions of US dollars and working to oust reformist ministers.
Tehran, which initially supported the US drive to unseat Afghanistan's Taliban regime, denies it is working against President Hamid Karzai's Western-backed government.
In a document dated March 2009, US military intelligence said a group of more than 100 Afghan and foreign Taliban had travelled from Iran to Afghanistan to launch suicide attacks.
In May the same year, General Stanley McChrystal, then US and NATO commander, said according to the documents: "The training (of insurgents) that we have seen occurs inside Iran with fighters moving inside Iran."
A threat report dated February 2005 alleged Taliban leaders in Iran were planning attacks in Helmand and Uruzgan provinces.
"The leaders travel into Afghanistan to recruit soldiers," said the report. It added the Iranian government had offered each leader about 1,740 dollars for any Afghan soldier killed and 3,480 dollars for any government official.
Another report from January 2005 said that Iranian intelligence services paid 10 million Afghanis (212,000 dollars) to the Hezb-e-Islami Gulbuddin rebel militia.
A statement sourced to "human intelligence" in June 2006 said Iranian officials were training members of the Taliban and Hezb-e-Islami Gulbuddin in Birjand, Iran.
Bombs and vehicles for suicide bombers were sent into Afghanistan from there, the same report said, while two other reports also spoke of bomb-making equipment coming from Iran.