Gates told a news conference at the presidential palace that he and Karzai had discussed the Iranian weapons issue.

"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."

The Iranian weapons are being supplied to the Taliban insurgents, he said, adding that some may also be headed to criminals involved in Afghanistan's drug trade. Gates did not specify what types of weapons were involved.

A Nato spokesman told reporters last week that a powerful type of roadside bomb like those used in Iraq, but never before seen here, was found recently in Kabul. The bomb, known as an EFP, or explosively formed projectile, was notable for its level of sophistication.

Asked separately whether he believed Tehran was behind the flow of weapons, Karzai suggested it was unlikely.

"There's no reason that any of our neighbours should support the Taliban," Karzai said. "We don't have any such evidence so far" regarding possible Iranian government involvement, he said, adding that relations between the two nations were improving.