The Iranian Revolutionary Guards have lost 13 troops in Syria in recent days with 21 others wounded, Iranian media reported .
It was Iran's biggest loss of forces within such a short time, based on official figures.
The names of those killed and when their remains will be repatriated will be announced later, the Guards said on Saturday.
All were from Iran's northern province of Mazandaran, Hossein Ali Rezayi, a Guards spokesman in the region, told the ISNA and Fars news agencies.
The deaths and injuries occurred in Khan Touman village just south of the city of Aleppo, the official IRNA news agency reported a Guards statement as saying.
Pro-government troops had driven rebels out of Khan Touman in December, but on Friday rebels led by the al-Nusra Front captured the strategic village.
Around 30 pro-regime troops were killed in the battle said the Britain-based Syrian Observatory for Human Rights which relies on a network of sources in Syria.
Iran is Syria's main regional ally, sending financial and military aid, including military advisers and volunteer forces from Iran, Afghanistan, Iraq and Pakistan, to prop up President Bashar al-Assad's regime.
Dozens of Iranian "advisers" have been killed in Syria since late 2015, including Revolutionary Guards commanders.
Saturday's news came as Ali Akbar Velayati, a top adviser to supreme leader Ayatollah Ali Khamenei, met Assad in Damascus and assured him of Tehran's support.