The conference on Thursday is expected to discuss a framework for NATO forces to gradually hand over security responsibilities to Afghan authorities.
Iran is considering attending a summit in London this week on Afghanistan but must first be convinced that the West is prepared to take a regional approach to stabilising the country.
The conference on Thursday is expected to discuss a framework for NATO forces to gradually hand over security responsibilities to Afghan authorities.
"If a regional approach is adopted towards the resolution of Afghanistan issues, if this plays an important role at the London conference, it would be grounds for Iran to attend," foreign ministry spokesman Ramin Mehmanparast said at a weekly press briefing.
"If we fail to reach the conclusion that the London conference approach would lead to the resolution of Afghanistan issues, it is unlikely we would take a positive view on participating."
Western powers want regional players to cooperate in bringing stability to a country where US and other foreign troops back the government of president Hamid Karzai in the face of an insurgency by the Islamist Taliban.
Afghanistan's neighbours, including Iran, met in Istanbul on Tuesday to find a "single voice" in their approach to the conflict ahead of the London conference. But Iran has not said definitively that it will attend the Thursday meeting.
Karzai is expected to give details of a programme to reach out to Taliban insurgents as part of a political settlement.
Mehmanparast said the presence of foreign troops in Afghanistan was a key reason for the problems in its eastern neigbour.
Western forces have been in Afghanistan since 2001, when the United States led an invasion to drive the Taliban from power over their alliance with al Qaeda.
"Military intervention has been wrong from the beginning. Terrorism was not uprooted, narcotics were not restricted and other problems in Afghanistan were not solved," he said.
"Narcotics multiplied during the presence of British forces and they should answer why this was."
Iran is at odds with Britain over its nuclear energy programme, which Western powers fear is aimed at acquiring nuclear weapons. Tehran says it is only interested in electricity.
Tehran has also accused Britain of backing opposition protests after presidential elections last June that have plunged the country into turmoil. Iran says it is reviewing its ties with Britain.
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