Several Iranian soldiers were killed near Aleppo, the Revolutionary Guards said on Saturday, in what appeared to be one of Iran's biggest losses in Syria since it deployed forces to support Syrian President Bashar al-Assad.
Islamist insurgents on Friday seized the strategic village of Khan Touman, some 15 km (9 miles) southwest of Aleppo. Dozens of people were reported to have been killed in the battle.
Fars news agency on Saturday quoted a Revolutionary Guards official as saying that 13 Iranian military advisors had been killed and 21 wounded in the fighting.
The attack on Khan Touman was launched by an alliance of Islamist insurgents known as Jaish al-Fatah, including the al Qaeda-linked Nusra Front, which has rejected diplomatic efforts to halt the five-year civil war.
Earlier this week, the United States and Russia brokered a ceasefire in the city of Aleppo itself. But fighting in the countryside to the south of the city, one of the major fronts where Iranian-backed forces have been deployed, has escalated in recent days.
The Syrian Observatory for Human Rights, a British-based group that monitors the conflict, said that Syrian and Russian jet planes have intensified their bombing of insurgent posts.
Jaish al-Fatah and affiliates have published on social media videos and photos of what appear to be the bodies of Iranians or other Shia militias who were killed in Khan Touman.
Some videos include footage of their wallets, personal documentation and Iranian currency.
There are indications that some of the casualties might be Afghans who are trained in Iran and are deployed alongside Iranian soldiers in Syria.
The Revolutionary Guards in a statement on Saturday urged people not to be affected by the Islamist rebels' "psychological war on the social media".
A top advisor to Iran's Supreme Leader on Saturday reiterated Tehran's continued support of Syria's president in a meeting with Bashar al-Assad in Damascus.
"Iran will use all its means to fight against terrorists who are committing crimes in the region," Ali Akbar Velayati, Ayatollah Khamenei's top adviser on international affairs, was quoted as saying by the Fars news agency.
The war in Syria has killed more than 250,000 people though, with tens of thousands unaccounted for, some say the death toll may be as high as 400,000.