MUSCAT, Oman - 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 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.

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

"This is an important thing to note, the direct implication of foreigners fighting on Syrian soil now for the regime," the official said.

Kerry is on a dual mission in the Middle East this week to foster political talks between Assad's resurgent regime and the embattled rebels, and to inaugurate a new round of peace talks between Israel and the Palestinians.

The State Department official said the Syrian opposition, which is badly split, has not finalized its representative to the talks Wednesday in Jordan.