A general in Iran's Revolutionary Guards was killed in Syria on Thursday during fighting between Syrian government forces and jihadists, an Iranian news agency reported.
Tehran has been a staunch ally of Syrian President Bashar al-Assad during the civil war that broke out in 2011, providing military advisers.
The worst fighting in years broke out in northern Syria this week between jihadists and the army.
"General Kioumars Pourhashemi, one of Iran's senior advisers in Aleppo, was killed in an attack carried out by takfiri terrorist mercenaries," Tasnim news agency reported.
Hayat Tahrir al-Sham (HTS), led by Al-Qaeda's former Syria branch, launched a surprise attack on the army in Aleppo province on Wednesday, the Syrian Observatory for Human Rights said.
The fighting has left more than 200 combatants dead, the Britain-based war monitor said.
The Revolutionary Guards confirmed the death of a "military adviser in Iraq and Syria", identifying him as a veteran of the Iran-Iraq war of the 1980s.
Iranian Foreign Minister Abbas Araghchi said the flare-up in Syria was part of a US and Israeli "plan to disrupt the stability and security of the region".