Iranian weapons have been found in Afghanistan, US Defense Secretary Robert Gates said on Monday, but he and Afghan President Hamid Karzai have agreed that Tehran involvement cannot yet be proved.

“There have been indications over the past few months of weapons coming in from Iran,” Gates said. “We do not have any information about whether the government of Iran is supporting this, is behind it, or whether it’s smuggling, or exactly what’s behind it.”

It is suspected that the weapons from Iran are being supplied to Taliban insurgents or criminals involved in the drug trade. The type of weapon involved has not been specified.