With official visits overlapping in Afghanistan, Iran's outspoken president and the US defence secretary have accused each other of interfering in local affairs.

While paying his first visit to Afghanistan, President Mahmoud Ahmadinejad of Iran lashed out at the United States, accusing it of playing a double game in Afghanistan.

On an invite to the country by President Karzai, the Iranian premier said the US was an unwelcome force in Afghanistan and no solution for peace.

Ahmadinejad told a joint news conference with Karzai that the US was being duplicitous in its treatment of Afghanistan.

He said: "Why is it that those who say they want to fight terrorism are never successful? I think it is because they are the ones who are playing a double game. They are the ones who set the terrorists on their course and now they say: 'Now we want to fight them.' Well they cannot, it is impossible. Our policy is full support for the Afghan people and Afghan government and reconstruction of Afghanistan and we will continue this support in the future."

As an aside directed at the US defence secretary, Ahmadinejad posed the question: "What are you even doing in this area? You are from 10,000 kilometres over there. Your country is on the other side of the world. What are you doing here? This is a serious question."

US Defence Secretary Robert Gates, in Afghanistan to review the surge of US and NATO troops, had said earlier in the week that Iran was playing a double game in Afghanistan by being friendly to the government while trying to undermine the United States.

He said on Wednesday he had shared his concerns with President Karzai.

Ahmadinejad has repeatedly played up Afghanistan's close ethnic and religious ties to Iran, while US officials have long accused Iranian political figures of maintaining links to Islamist insurgents in Afghanistan.

Gates left Kabul shortly after Ahmadinejad landed and was unable to immediately respond to the Iranian leader's comments.