Thirteen Iranian soldiers were killed in a battle with Islamist militants over a village near the Syrian city of Aleppo, Tehran said on Saturday, 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 15 km (9 miles) 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 quoted a Revolutionary Guards official as saying that 13 Iranian military advisors had been killed and 21 wounded.
The British-based Syrian Observatory for Human Rights said it had confirmed from its sources on the ground the death of 20 Iranians, including 13 advisors.
The monitor said that among the Iranian-backed militia fighters involved in the battles, six from Lebanon's Hezbollah Shi'ite movement and 15 Afghan Shi'ite fighters were also killed.
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.
Iran, 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.
A senior advisor to Iran's Supreme Leader on Saturday reiterated Tehran's continued support of Assad in a meeting with the president 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 adviser on international affairs, was quoted as saying by Fars.
Iranian proxies, including Afghans and Iraqis as well as Lebanese, have been involved in Syria from as early as 2012.
While Tehran previously said its support was limited to advisors, it has been more open about the extent of its role since Russia intervened on Assad's side last year.
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 1 a.m. on Saturday (2200 GMT on Friday), Syrian state news agency SANA reported.
The Syrian Observatory for Human Rights, a British-based monitoring organisation, said Syrian and Russian warplanes had intensified their bombing of insurgent positions near Khan Touman.
In northern Syria, U.S.-led coalition airstrikes killed 48 Islamic State fighters on Saturday, Turkey's state-run Anadolu Agency said, quoting the Turkish military.
The strikes were in response to increasingly frequent Islamic State attacks against opposition forces in the area, Turkisk security sources told Andadolu.
The Turkish border town of Kilis, which lies just across the frontier from Islamic State-controlled territory of Syria, has been hit by regular rocket fire in recent weeks.
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.