Iranian President Mahmoud Ahmadinejad has denied US allegations that Iranian weapons are finding their way into the hands of the Taleban.
Mr Karzai (L) has been positive about Mr Ahmadinejad 's role
Mr Ahmadinejad, on a visit to Kabul, said that Tehran strongly supported the political process in Afghanistan.
He held talks with his Afghan counterpart, Hamid Karzai, on the first leg of a tour that will also take him to Turkmenistan and Kyrgyzstan.
This is Mr Ahmadinejad's first visit to Afghanistan since he was elected.
Kabul's main ally, the United States, recently alleged that Iranian-made weapons were reaching the Taleban.
"The same allegations are made in Iraq. They are saying that they discover some weapons," Mr Ahmadinejad said at a press conference he addressed jointly with President Karzai.
"What is the reason? Why are they saying such things? Iran is a big country. I have serious doubts about this issue," he said.
"With all our force, we support the political process in Afghanistan," Mr Ahmadinejad said.
He added that Iran's security depended on the situation in Afghanistan as the two nations shared a long border.
"For us, a secure and stable Afghanistan is the best," he said.
President Karzai said Afghanistan's ties with Tehran were excellent and addressed the Iranian president as his brother.
Mr Karzai said his country was close to Iran and also a friend and strategic partner of the United States.
"If Afghanistan can bring them [Iran and the United States] closer, that will be a great happiness for Afghanistan - but it depends on both sides," he said.
During his recent visit to the US, Mr Karzai said Iran had been playing a positive role in his country, putting him in dispute with President George W Bush.
Tehran helped to establish an interim government in Kabul after the fall of the Taleban and since then has donated large amounts of aid to Afghanistan.
Issues of mutual interest include continuing reconstruction efforts in Afghanistan and Kabul's ongoing fight against Taleban insurgents, says the BBC's Pam O'Toole in Tehran.
The continuing presence of almost one million Afghan refugees in Iran and Tehran's current operation to expel other groups of Afghans who are living in Iran illegally are also key issues.
From Afghanistan Mr Ahmadinejad will travel on to Turkmenistan, where he is expected to sign a number of co-operation agreements.
Mr Ahmadinejad's final port of call will be a meeting of the Shanghai Co-operation Organisation in Kyrgyzstan, where he is expected to bid for Iran's status to be raised from observer to full member.
While there he is due to hold talks with leaders of full members of the organisation, who include Russia and China.