''In Siria, there are over 60,000 fighters from Iran," said Ahmad Jarba.
For this reason Iran should not attend Syrian peace talks dubbed Geneva II scheduled to take place next month, Jarba said.
"Iran cannot take part in 'Geneva II' while it is killing the Syrian population," he told Arab and western foreign ministers gathered in London to meet Syrian opposition officials.
"Assad's regime would already have fallen were it not for the Iranian presence in Syria," Jarba claimed.
The Friends of Syria group comprises Britain, Egypt, France, Germany, Jordan, Italy, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States.
Iran as yet has no role in Geneva II, but Britain's foreign secretary William Hague said on Tuesday he was trying to use new positive diplomatic relations with Iran to encourage it to play a "more constructive role".
The Syrian National Coalition will not attend the Geneva II talks until their objective is to get Assad to step down, Jarba said.
''The sultan has to go," he stated.
More than 100,000 people have been killed in fighting between rebels and pro-Assad forces since the Syrian conflict began in 2011.