Thirteen Iranian soldiers have been killed in a battle with Islamist militants over a village near the Syrian city of Aleppo, Tehran says, in one of Iran's biggest single-day losses since it sent forces to support President Bashar al-Assad.
Russia meanwhile said that a truce in Aleppo itself had been extended until Monday.
Islamist forces seized Khan Touman village, about 15km southwest of Aleppo, on Friday and dozens of people were reported to have been killed in the fighting.
The attack was launched by an alliance of Islamist insurgents known as Jaish al-Fatah, including the al Qaeda-linked Nusra Front.
Iran's Fars news agency on Saturday quoted a Revolutionary Guards official as saying that 13 Iranian military advisors had been killed and 21 wounded.
Jaish al-Fatah and affiliates posted videos and photos on social media of what appeared to be the bodies of Iranians or Shi'ite militiamen who were killed in Khan Touman.
They included footage of wallets, personal documents and Iranian currency.
Tehran, along with Russia, has been a principle ally of Assad in the five-year-old civil war, while Gulf Arab states and the West have supported various rebel factions.
Iran has been particularly involved in campaigns around Aleppo in northwest Syria, which was the country's commercial and industrial centre before the war and is now divided between government and rebel forces.
Fighting in the countryside to the south of Aleppo has escalated in recent days despite a ceasefire in city itself since Wednesday.
The Russian defence ministry said a "regime of calm" truce in Aleppo and parts of Latakia province had been extended for 72 hours beginning at 1am local time on Saturday, Syrian state news agency SANA reported.
