Iran has sent soldiers to Syria to fight alongside forces loyal to President Bashar al-Assad and those of the Lebanon-based Hezbollah militia, a senior State Department official said Tuesday.

An unknown number of Iranians are fighting inside Syria, the official said, citing accounts from opposition forces supported by Western and Arab governments. The official spoke on the condition of anonymity to preview a strategy session that Secretary of State John F. Kerry will hold Wednesday with key opposition supporters.

Syrian rebels have claimed for weeks that Iran is sending trained fighters to the Syrian civil war, and Iran-backed Hezbollah has said baldly that it will not let Assad fall