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 in Syria, the official said, citing accounts from members of the opposition Free Syrian Army, which is backed by the United States. The official spoke on the condition of anonymity to preview a strategy session that Secretary of State John F. Kerry is to hold Wednesday with key supporters of the Syrian opposition.

Rebel forces have alleged for weeks that Iran is sending trained fighters to Syria, and the Iran-backed Hezbollah has said baldly that it will not let Assad fall.

But with the British, French and American governments considering providing arms to the Syrian opposition on a scale not yet seen in the civil war, the U.S. official’s allegation was a tacit acknowledgment that the two-year-old Syrian conflict has become a regional war and a de facto U.S. proxy fight with Iran.